Add custom date histogram interval option

[lukasolson]

This adds a new option when selecting interval for a date histogram that allows you to enter a custom interval.

Closes #1784.

[simianhacker]

When you set custom interval to '0.5d' then it crashes Chrome.

http://localhost:5601/#/visualize/create?_a=(filters:!(),linked:!f,query:(query_string:(analyze_wildcard:!t,query:'*')),vis:(aggs:!((id:'1',params:(),schema:metric,type:count),(id:'2',params:(customInterval:'0.5d',extended_bounds:(),field:'@timestamp',interval:custom,min_doc_count:1),schema:segment,type:date_histogram)),listeners:(),params:(addLegend:!t,addTooltip:!t,defaultYExtents:!f,mode:stacked,shareYAxis:!t),type:histogram))&indexPattern=%5Blogstash-%5DYYYY.MM.DD&type=histogram&_g=(time:(from:now-7d,mode:quick,to:now))

[simianhacker]

LGTM