Create new .netrc file when not present

[shankardevy]

When .netrc file is not present in the user's home directory, trying to save a new entry triggers FileNotFoundError.

Can this library automatically create one when .netrc is not found?

[sloria]

Hm, the netrc from the stdlib upon which this is built raises an error if .netrc doesn't exist. I'd like to keep this lib consistent with that.

However, perhaps we could add a parameter to force creation if it doesn't exist:

Netrc(create=True)

I would welcome a PR for this.

[sloria]

Closing this for now since I won't be working on it in the near future.