Add entry for file names

Fully qualified module names are nice, but sometimes you just want to jump to _Filename.hs_ somewhere in your project tree. This patch adds tags for the basenames of the files as they are encountered, so you can do

    :tag Connection.hs

and have it pull up _src/Network/Http/Connection.hs_ which is a nice complement to pull #9 where you can do

    :tag Network.Http.Connection

AfC