Improve DashKeywords

Calling DashKeywords without arguments will show the keywords set for
the current buffer.
Typo
Add default keywords for searching withing Dash

Creates a list of mappings for docsets and filetypes known to vim, which
will let most users automatically search a group of their installed
docsets whenever they are in a recognized filetype.

This list will be following the descriptions in accordance with:
http://kapeli.com/dash_plugins
Always adds new keywords instead of resetting
Take into account versioned docsets
Move defaults to their own file
Merge branch 'develop' (fixes #17)