Set channeldir to other directory

[beodegit]

Hi,

it would be nice if the directory where the channel xml files are stored (channeldir) could be user-defined, so it is set to a directory different than $HOME/.castget/.
This could be done in the castgetrc file.

Thanks.

[mlj]

This should be combined with changing the default locations for the configuration file and the channel files to standard XDG locations. Command-line overrides would also be nice. But we will have to work out the exact search order and the priority of the different settings first.

[jetmore]

Based on comment in #6 (comment), should be associated with and possibly merged with #58