Allow passing a .gitignore file with a flag like --ignore-file

[casey]

Adding the ability to pass a .gitignore file to imdl torrent create would allow easily creating a torrent from a git repo:

$ imdl torrent create --input repo --ignore-file repo/.gitignore

I think, but am not totally certain, that globset, the library intermodal uses for --glob parsing, takes the same syntax as .gitignore. If so, this could be easily supported.

[casey]

Done by @Celeo! You still can't pass a custom ignore file by flag, but I think this satisfies the most important use case of this feature. If you want to be able to pass a new ignore file by flag, ping this issue or open a new one! It would be very easy to add, now that we're using the ignore crate.