Bump splainer to 2.20.1, add custom headers support for ES/OS

[worleydl]

[epugh]

Could we maybe add some docs right above the box to say what this is for? I.e, if you are using a apikey?

Or, should we force you to put in your api key below the whole query area?

[epugh]

Change it up to add a apikey at the splainer landing page level., but in the detail have a drop down.

Check on the solr custom headers box showing up ;-)

[epugh]

@worleydl I think you need to commit the customHeaders file ;-) I don't have it!

[epugh]

Looks like we need both the directive and the controller! GEtting close!