Adding xbps_fetch_file_dest().

This function is similiar to xbps_fetch_file(). In contrast to xbps_fetch_file()
xbps_fetch_file_dest has an extra paramenter which allow to define an output file
for the request.
xbps-uhelper fetch supports user defined output file

xbps-uhelper now supports downloading files with a defined destination.

The syntax of fetch is "URI>FILENAME". So if you call

	xbps-uhelper fetch "http://foobar.com/foo.tar.gz>bar.tar.gz"

the output file will be named "bar.tar.gz"