Support default filters

[ericholscher]

It would be very useful to have the ability to have a default search query that users get when they open. This would be configurable at the integration point for the extension, perhaps adding a config option:

rtd_sphinx_search_default_filters = 'subprojects:docs'

The use case here is:

• I am a project on RTD that has subprojects
• We want users to be able to search across all the subprojects easily
• When they launch search, it should already have subprojects:<slug> prepended to the search in some fashion