Skip unknown escape sequence commands

Many ANSIs in the AWAKE artpack by Blocktronics contains the following
escape sequence:

  <escape>[7h

As Escape.js just skips it, lets do the same.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Skip unknown graphics mode arguments

The "70" graphics mode argument appears in some ANSIs in the AWAKE
artpack by Blocktronics. Just skip them like escape.js does.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Add support for viewing multiple files

This adds support for viewing multiple files to make browsing an artpack
easier.

Launch with:

  ./duhview ../artpacks/*.ans

and then use the Enter key to switch to the next ANSI.

Signed-off-by: Pekka Enberg <penberg@kernel.org>