add filetype command support ...

This adds support for a filetype detection command. It can be
used to print a filename on the console or to set a variable
with the detection result for use in shell scripts.
The command also has an option to print the known filetypes
so that a user knows what to match for.

Based on an earlier version from Jean-Christophe.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>