feat: Adds support for header (--header) configuration based on the spec.

[jbottigliero]

• Adds shim to ensure fallback to the current default (not the value in the spec)
• Adds warning for --changelogHeader usage.

joe:standard-version $ standard-version --dry-run --changelogHeader="Hello, World"
[standard-version]: --changelogHeader will be removed in the next major release. Use --header.

[coveralls]

Coverage increased (+0.005%) to 99.429% when pulling 9d51ff1 on feat-header-support into bc606f8 on master.

[jbottigliero]

Resolves #364

[bcoe]

@jbottigliero is the default behavior the same headers as before? just want to make sure we don't break a bunch of CHANGELOGs.

[bcoe]

(to be fair, we look for the start of a version ## 3.x.x to decide on where the last CHANGELOG entry was, so I don't think changing the header would be likely to break anyone).

[jbottigliero]

This shouldn't change any of the behaviors, just the sourcing... I did have to move the validation that was occurring in the .check into the actual command handler in order to allow backward compatibility and proper overriding of the flag(s).

Let me know if you think there is a specific case missing in the suite to add additional coverage.