Clean up handling of ~/.vim/assume-iterm

This is a "signal" file left in the filesystem to modify the behavior of
my ~/.vimrc. It varies per machine and isn't intended to appear in
source control.

Make it a dot-file while we're at it, to keep it out of standard
directory listings.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Git: make untracked files blue in `git status` output

Better visual separation between the main file statuses. Blue was chosen
because it appears "cool", and leads to an association with "icebox".

Signed-off-by: Wincent Colaiuta <win@wincent.com>