feat: support single-character flags

[mmkal]

Closes #26

Changes:

• removed the check against 1-character flagNames
• added a check against 1-character flags with an alias defined

Tests:

• removed the test making sure 1-character flags were disallowed
• added a test making sure 1-character flag names work
• added a test making sure 1-character flag names can be used in alias groups along with longer flags (git commit -am hello)

Note:

I was surprised it was as easy as this. And from playing around it seems that type-flag doesn't seem to care if you use -- or -, for flags of any length. For example foo --message hello and foo -message hello both seem to work. Likewise foo -m hello and foo --m hello.

I wasn't sure if this was deliberate, I couldn't find it in the docs. But either way, I wanted to get this in along with test cases. So if the above is considered a bug, not regressing support for single-character flags would be part of the fix requirements if this goes in first.

Another note:

For this to make its way to cleye, there might be some special-casing needed for the --help renderer (to avoid suggesting --x 1 usage instead of -x 1 (although both work as noted above)). I can help with that too if you'd like, once this or something like it is published.