README: update installation section
README: replace into sentence with header elaboration
README: actually let's make that a subheader
Cleanup ternary in case_regex
README: add screenshot section
Make command name structure more vim idiomatic

PluginCommand rather than PlugInCommand.
Factor out <Plug> bindings
Pivot default mapping
Match header tagline to README
Belatedly bump the version string
Refactor patteren handling in set_match
Escape /s in set_match
Factor set_{1,2,3}match out of set_match
Push down cased_regex call site
Refactor used_{1,2,3}match into a dictionary
Lazy define s:matches
Carry matches locally on each window rather than once in script
README: tweak usage screenshot caption
README: breakout note about preventing default mappings
README: rework basic usage section