Add Perl syntax highlighting to alienfile

[plicease]

alienfile is a recipe used by Aliens during the build process. alienfile, like cpanfile is essentially Perl source. It is already in use by a few Alien developers aside from myself. It would be helpful, and I think not harmful to get the same syntax highlighting as cpanfile.

[zmughal]

Does this need a rebase?

[oalders]

@zmughal, I think it does need a rebase. @haarg do you have any opinions on these changes? I'm inclined to merge unless there are objections.

[haarg]

This should be fine after a rebase.

[plicease]

rebased on master.

[oalders]

Thanks @plicease!