Add command to run commands in sequence

[kastiglione]

This introduces the sequence command (seq for short), which allows multiple commands as a single command.

Example A:

breakpoint command add -o 'seq bt; po someVar'

Example B:

command regex b 's/(.+) if (.+)/seq _regexp-break %1; breakpoint modify -c "%2"/' 's/(.*)/_regexp-break %1/'