Rename the plugin

[jaraco]

I don't care for the fact that the plugin must be referred as "plugin-enabled options". That probably should be just "enabler". I'm considering renaming that, but unsure when or how, so I'll await some feedback here first.

Originally posted by @jaraco in #10 (comment)

[jaraco]

The reason for naming it "plugin-enabled options" was to give a more descriptive name to the entry point. I hadn't realized at the time that the name of the plugin as defined by the entry point was exposed to the user, so to disable the plugin, one has to use -p "no:plugin-enabled options" and use quotes to ensure proper parsing.

The plan instead is to change that to simply "enabler", following the convention that most pytest plugins use, which is to reflect the pytest-stripped name.