Support gd / gD

[achalddave]

It'd be cool if there were also functions to override gd/gD to override smartcase.

Right now there is also some issue with using the keeppos feature and gd/gD. Specifically, if you

1. position yourself like so 'vim-asterisk'
2. hit *
3. cancel that search
4. go back to 'vim-asterisk', position yourself as 'vim-asterisk'
5. hit gd

The gd search will use the regex used to position you at 'vim-asterisk'.