Allow mu4e-bookmarks to contain lisp expressions

This allows expressions such as:
```
((concat "maildir:/inbox date:"
	 (format-time-string "%Y%m01"
			     (subtract-time (current-time) (days-to-time 28)))
	 "..")
 "Last calendar month"
 "m")
```
which allow me to look at the last calendar month (i.e. starting from the 1st of the month).