Handle alternate font via `-F` and `-S`

New command-line arguments are introduced to specify an alternate font.
The alternate font parts can be specified in the strings by surrounding
the relevant substrings with '\(' and '\)'.

E.g.:
    textwidth -f sans-serif -s 10 -F serif -S 13 'foo\(bar\)baz'
In the above example, the returned width will be:
    tw('sans-serif', 10, 'foobaz') + tw('serif', 13, 'bar')