Add --interactive flag

[alicederyn]

It would be very convenient for rex to offer an --interactive mode, where it enters a REPL for bash that captures all commands run and, once the user has run exit or hit ^D, drops into an editor for a commit message with all evaluated commands listed in a script block.

We can probably leverage bash itself for the REPL, if we can reliably configure its history mode without the user's config files interfering. This means:

• Changing which file history gets written to
• Turning off the leading-space-disables-history feature
• Setting a trap for errors that documents ways to recover (e.g. using || true)

We will also ideally have some consistent way of handling executed git commands in a non-surprising way; for instance, we could do one or more of:

• strip informational commands like status from the history
• replace the command itself to intercept and disable commands like commit
• leave the repository locked to prevent concurrent operations