Update tag to accept the -g flag.

[YPCrumble]

@aykamko this resolves #23 to allow passing the -g flag. It returns the tagged filename, and then editing that file (e.g., using the alias e1) would take you to the first line of the file in vim.

Per your suggestion the changes don't impact any environment variables. using the --notag flag still returns the untagged filenames without issue.

Let me know what you think? It's my first commit to a golang repo so any advice feedback would be welcome.

Thanks again for building this!

[aykamko]

Hey @YPCrumble, thanks for working on this! I'm back from the dead and would be happy to get this merged, but it's gonna need a little bit of rework since I've changed up how parsing works.