Fix various spelling / grammar / consistency errors
Fix one more grammar error
README: fix missing guard around liquidprompt for bashrc instructions

Fixes https://github.com/nojhan/liquidprompt/issues/291
liquidpromptrc-dist: fix wrong quotes
Miscellaneous grammatical & consistency changes

- Various grammatical changes and rephrasings
- Capitalize names of VCSes and "Bash"
- Standardize references to project name ("Liquid Prompt")
- Use title casing in README section headers
- Use multiple list-levels in README where appropriate
- Fix code block in pull request instructions in CONTRIBUTING