Inputs for default (fallback) author name/email

[petetomasik]

Fixes #70

GitHub events like status don't include a pusher field in their payload, causing the action to send empty author information to the Buildkite API. This results in builds being attributed to the API token owner instead of the actual commit author.

Changes

• Added two new optional input parameters: commit_author_name and commit_author_email
• Refactored author extraction into separate get_author_name() and get_author_email() functions
• Added tests
• Updated documentation to explain new processing order

New processing order

1. .pusher.name and .pusher.email (existing behavior)
2. .head_commit.author.name and .head_commit.author.email (for push events)
3. .commit.commit.author.name and .commit.commit.author.email (for status events)
4. commit_author_name and commit_author_email input parameters (user-provided)
5. Git commit information from the repository

NOTE: Both name and email are processed as separate values. This means that if only one of commit_author_name or commit_author_email is provided as an input parameter the other value could resolve to a different "author".

[jeremybumsted]

nice one!