[cygwin][idfile] New function "iwhich" to tell the command is alias/function/executable.
[cygwin][vim] 1. quick vi alias. 2. improve vim statusline. 3. f12 for git view.

1. vi for quick startup; vim to keep full vim features
2. vim statusline: ranbow and syntastic passive/active mode
3. f12: switching between gitgutter and gblame
Merge remote-tracking branch 'origin/cygwin' into freebsd91p7