air-email-review: add git credential helper support

[horms]

air-email-review accepts an optional token, passed on the command line or configured in the configuration file. In both cases, in plain-text.

As a credential of sorts it seems useful to allow it to be configured other than using plain-text. And the proposal here is to add support for using git credential helpers. In a similar way to git itself.

Link: https://git-scm.com/docs/gitcredentials

[horms]

My plan is to use implement a subset of the behavior described at the link above. Sufficient to configure helpers to retrieve the token.

I do not plan to initially support:

• configuring a helper as a bash snippet
• configuration file syntax for scoped helpers ([credential URL] syntax)