Parses "to:" address for signal to add waiting label

To auto add a "wait X (days|weeks)" label, just include (To, CC, or BCC) yourself on the email, adding "+ X d" to your own email (e.g. "mark+1d@example.com"). Recognizes "d", "day", "w", "wk", and "week" (plus or minus trailing "s"). Closes #2. Also adds setup() function to add required top-level labels.