Refactor patterns; add tests, better blanks support

[eykd]

I apologize that this is all in one commit. I got going and the next thing I knew, I had gone too far without a commit.

Some highlights:

• Compile patterns in poyo.patterns instead of _Parser.__init__. Compile flags are important contextual information that belong with the patterns themselves.
• Add tests for compiled patterns.
• Add better blank line support. The original impetus! Previously, blank lines in certain places, such as immediately after a section start or inside a list, would confuse the parser and raise an error. I've added support for blank lines in these situations.
• Moved test YAML for parser test inside of test module. It wasn't really necessary to have since the parser only works with strings anyway, and it seemed easier to work on the test with everything in one place.

The blank line support probably isn't perfect yet, and the new LIST pattern has grown rather hideous, but the tests pass and I think it's a good start.

[eykd]

Closes #4. 😁

[hackebrot]

Thank you for this PR @eykd 🙇

Hope I'll get it to merge this one in in the next couple of days. There are a few changes I'd like to make to it though.

• I agree that the compile flags belong to the patterns. For the sake of readability, I'd prefer to rename the string patterns rather than the compiled ones for better imports
• Tests are awesome, I'll add some pytest touches to it. Using parametrized fixtures over loops will get us better test result reports
• Didn't know blank lines in list values are a thing 😁 Not a big fan of YAMLs flexibility in that particular case, great to be able to support it though
• I like to keep the YAML string in a separate file to run a local script to compare the parsed dicts of poyo against ruamel.yaml. Also I like syntax highlighting on the YAML text, which gets lost in Python code