Improve formatting and grammar of README

This makes README.md use more of the features of Markdown so that it will be easier to read on https://github.com/hchbaw/opp.zsh. I also fix some grammar and capitalization errors.
Update the docs; no code changes

Simply delete the contents which I feel it could be deleted.

Signed-off-by: Takeshi Banse <takebi@laafc.net>
Opp 0.0.8

Signed-off-by: Takeshi Banse <takebi@laafc.net>
wip: shellargument

Signed-off-by: Takeshi Banse <takebi@laafc.net>
wip: handle zle results

TODO: opp-generic-w

Signed-off-by: Takeshi Banse <takebi@laafc.net>
Teach underscore (_) as iskeyword-ish

Thank you very much pointing me on #4, martong!

Signed-off-by: Takeshi Banse <takebi@laafc.net>
Merge branch 'tb/textobj-shellargument' into thb

* tb/textobj-shellargument:
  wip: shellargument
Merge branch 'tb/zle-results' into thb

* tb/zle-results:
  wip: handle zle results