Implement wifi-ish table for linux

[blaedj]

This commit:

• adds a simple 'string-delimited' strategy for dataflatten, including
  record-splitting based on the idea that if we see a key 'again', we
  should start a new record, instead of over-writing the key.
• adds a new KeyValue data source type to dataflattentable
• adds options to the TablePluginExec function
• implements a wifi table via a dataflattentable.TablePluginExec call

Note that this table will only work if nmcli is installed, so it's not a true
solution for all linux installations.

osquery> select * from kolide_nmcli where key not in ('BSSID', 'SSID', 'SSID-HEX');
+-------------+--------+-----------+-------------------------------------------------+-------+
| fullkey     | parent | key       | value                                           | query |
+-------------+--------+-----------+-------------------------------------------------+-------+
| 7/CHAN      | 7      | CHAN      | 11                                              |       |
| 7/RATE      | 7      | RATE      | 130 Mbit/s                                      |       |
| 7/BARS      | 7      | BARS      | ▂▄▆█                                            |       |
| 7/WPA-FLAGS | 7      | WPA-FLAGS | (none)                                          |       |
| 7/NAME      | 7      | NAME      | AP[8]                                           |       |
| 7/MODE      | 7      | MODE      | Infra                                           |       |
| 7/RSN-FLAGS | 7      | RSN-FLAGS | pair_ccmp group_ccmp psk                        |       |
| 7/SIGNAL    | 7      | SIGNAL    | 90                                              |       |
| 7/DEVICE    | 7      | DEVICE    | wlp2s0                                          |       |
| 7/FREQ      | 7      | FREQ      | 2462 MHz                                        |       |
| 7/SECURITY  | 7      | SECURITY  | WPA2                                            |       |
| 7/ACTIVE    | 7      | ACTIVE    | no                                              |       |
| 7/IN-USE    | 7      | IN-USE    |                                                 |       |
| 7/DBUS-PATH | 7      | DBUS-PATH | /org/freedesktop/NetworkManager/AccessPoint/127 |       |
| 0/SIGNAL    | 0      | SIGNAL    | 100                                             |       |
| 0/SECURITY  | 0      | SECURITY  | WPA2                                            |       |
| 0/DBUS-PATH | 0      | DBUS-PATH | /org/freedesktop/NetworkManager/AccessPoint/123 |       |
| 0/MODE      | 0      | MODE      | Infra                                           |       |
| 0/CHAN      | 0      | CHAN      | 6                                               |       |
| 0/BARS      | 0      | BARS      | ▂▄▆█                                            |       |
| 0/WPA-FLAGS | 0      | WPA-FLAGS | (none)                                          |       |
| 0/DEVICE    | 0      | DEVICE    | wlp2s0                                          |       |
| 0/FREQ      | 0      | FREQ      | 2437 MHz                                        |       |
| 0/RATE      | 0      | RATE      | 195 Mbit/s                                      |       |
| 0/RSN-FLAGS | 0      | RSN-FLAGS | pair_ccmp group_ccmp psk                        |       |
| 0/ACTIVE    | 0      | ACTIVE    | no                                              |       |
| 0/IN-USE    | 0      | IN-USE    |                                                 |       |
| 0/NAME      | 0      | NAME      | AP[1]                                           |       |

[directionless]

I like this. Some small scale things