Add unit tests for common Poedit headers.
Minor grammar/punctuation cleanup in unit test messages.
Add an option to print the results of the addtextdomain task to stdout
instead of updating files.

Toggled by adding --dry-run as a CLI arg.
Add docs for the new potHeader options.
Use the OS EOL character in addtextdomain tests.
Flip 'actual' and 'expected' arguments in makepot unit tests to match the docs.
Add unit tests to be sure expanded Poedit headers can be overridden.
Split the readme into separate files to make it easier to digest.