Added configuration file

This is the first step to make todotxt-machine configurable through a file. The most important thing (for me) was to be able to set the location of the todo.txt file, since the hard-coded default didn't suit me, and I do not want to call the program with a command line parameter every time.

Using this patch, the todo.txt file must be either specified by a command line parameter, or in the configuration file `~/.todotxt_machinerc`. The command line parameter overrides any configuration setting. The configuration is a simple `.ini` style text file that can be read by Python's `ConfigParser`, which is part of the standard library, as far as I know.

Colours or custom keybindings could be the next options added to the configuration file.