Added check git exist on exec some cli commands.

Fixed errors on exec some cli commands if git does not exist.
For example:

$ perl -I../lib minil new Test
Can't exec "git": No such file or directory at ../lib/Minilla/CLI/New.pm line 27.
Use of uninitialized value $username in substitution (s///) at ../lib/Minilla/CLI/New.pm line 28.
Can't exec "git": No such file or directory at ../lib/Minilla/CLI/New.pm line 30.
Use of uninitialized value $email in substitution (s///) at ../lib/Minilla/CLI/New.pm line 31

Added:
If git does not exist - displayed message 'The "git" executable has not been found.' and script terminates.