Named command args

[supersaiyansubtlety]

based on #277 rebased

Allows use of named args of the form some-name=some-value.
Particularly useful for commands with many optional args.

Unnamed positional arguments may still be used, even at the same time as named args, so long as all positional args come before named args.

Centralizes parsing of individual args into Argument and of commands' accepted args into ArgsParser.

Additionally, makes path verification more consistent and fixes the following bugs:

• breaking: InvertMappingsCommand used OUTPUT_FOLDER instead of MAPPING_OUTPUT
• InvertMappingsCommand args skipped an index
• DecompileCommand used getWritableFolder for OUTPUT_JAR
• users of ENIGMA_PROFILE didn't verify it was a file
• ComposeMappingsCommand didn't declare its optional de/obf args

[supersaiyansubtlety]

ready once #277 is merged

[supersaiyansubtlety]

rebased, ready for review

[ix0rai]

beautiful! i love how this fits into the old structure of command classes, and all the testing is great! just a couple issues with the messages and nitpicks.