Add more specific file pattern matching for fsharp

To avoid accidentally matching and applying our functions on .fsproj
files
Added BufEnter trigger to redisplay erors

BufLeave does a decent job of disabling error displays (not perfect, but
BufHidden doesn't seem to do what I thought it did).  But, once we
bufleave hide the errors, they never come back.  This BufEnter trigger
will cause them to redisplay.
Merge pull request #771 from sillyotter/master

Fixes for #763 and #762