Closes #740

… contributing sections

…esn't work when conflicts are also set

Closes #753

…ics if there were no subcommands

Closes #754

Flags, Opts, and Positionals now store their internals using compartmented Base, Valued, and
Switched structs to keep the code duplication down and make it easier to maintain.

Iniside the src/app/parser.rs there have been several changes to make reasoning about the code
easier. Primarily moving related sections out of the large get_matches_with into their own
functions.

… specific args vice command wide

One can now use `Arg::allow_hyphen_values(true)` which will enable `--opt -val` style values only
for the specific arg and not command wide.

Closes #742

Failing value validators now produce messages like the following:

error: Invalid value for '-j <val>': some message about the value

Closes #744