Write to gitignore

[lorepozo]

This PR implements issue #21, adding args for output instead of requiring bash redirection (note that you can still just use bash redirect).

• joe java > .gitignore
  == joe java --save
• joe java >> .gitignore
  == joe java --append
• joe java > /some/path/to/.gitignore
  == joe java --save -o /some/path/to/.gitignore

-a and -s can be used instead of --append and --save, respectively.

Includes commit from my styled PR #23 for ease of merge conflicts.

[karan]

I'm very skeptical about merging this - we want to keep the API extremely simple, and adding more args is not pushing us in that direction.

[syndbg]

I think @lukedmor 's idea isn't so bad. Currently if we write joe potato > .gitignore we'll get an error message written in the .gitignore.

If we use commands like --append or --save we have more control. We can decide if we want to write gibberish in the file or just display an error message in stdout.

However abandoning the existing behavior might be too much. If people like the simplicity of the API, then we should stick to that and just introduce something more advanced and less error-prone for people who like that.

[lorepozo]

As I've written it, no features have been removed. I only added those capabilities. I think I'm going to close this pull request though, so that the features can be merged later after other PRs are resolved. (namely, #23 and #31)

[karan]

We'll first handle #31, then #29 and then come back to this.

[lorepozo]

Updated the PR for the latest merges

[audunmo]

Why not have save be default behaviour for the case where ".gitgnore" not in os.listdir('.') == True? Seems to be that writing to stdout would be the less common thing to want to have happen.