Miscellaneous grammatical & consistency changes

There are quite a few changes in this pull request, but if any of the changes are too drastic or controversial, I can easily revert specific parts of the commit and then rebase. Here's a summary (also present in the extended commit message):

- 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`.

One thing I attempted to do was introduce consistency with respect to the way the name of the project is referenced. I opted for "Liquid Prompt" (referring to the name of the project as a proper noun) since `CONTRIBUTING` refers to the project in this way. Is this an appropriate change?