Add include and exclude section in config

[yngvar-antonsson]

I've added include and exclude sections to config to configure which metrics are exported.

When you add include section, only metrics from this section are exported:

metrics:
  export: ...
  # export only vinyl, luajit and memory metrics:
  include:
  - vinyl
  - luajit
  - memory

When you add exclude section, metrics from this section are removed from default metrics list:

metrics:
  export: ...
  # export all metrics except vinyl, luajit and memory:
  exclude:
  - vinyl
  - luajit
  - memory

I didn't forget about

• Tests
• Changelog
• Documentation (README and rst)

Close #222