xmltv: add a simple parser for episode number

Handling episode-num tag is very complex, as it supports a lot
of different formats and may have optional information, like
the number of seasons and if an episode is part of a bigger one.

Let's do a minimal parsing there, relying only on the
xmltv_ns episode num system.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>