Add configurability

[TomSeldon]

Although we purposefully do not want much configuration (as the goal is to standardise commit message format across our projects), I can see some areas that would be useful:

Allowed project short-codes:

Currently we just check that there's some text where it should be. In reality for a specific project you'll want to lock it down to only allowing commit messages with the specified project code. e.g. where a ticket is actually TESTPROJ-123 the commit may accidentally contain TESTPROJECT-123 which would be considered valid.

Example config:

# .commitmessagechecker.yml
project_shortcode_whitelist:
    - TESTPROJ

Allowed ticketing systems

Most projects will just use JIRA, some will also use Zendesk. I'm not sure if we use any others, but would be a good idea to:

• By default allow Z# and J#
• Allow this to be overridden per project using config

Example config:

allowed_ticketing_systems:
    - J
    - Z

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Anything else? We could allow commit types to be configured, but I think there's more value in enforcing consistency on that front.