Added missing quotes and made quote style consistent

As @rpdelaney has brought up on numerous occasions, we should be quoting our variables more often in case they contain whitespace. This PR aims to fix that.

In this PR:

- Add quotes to variables and subshells missing quotes in `test/prompt_test.sh`, a la #28 
- Add quotes to variables and subshells missing  quotes in `.bash_prompt`
- Adjust string that contain quotes halfway to quote entire string in `.bash_prompt`

/cc @rpdelaney for review