Add the file_size segment

Added the following functions to segments/vim.py:

* file_size();
* _to_human(size[, precision]).

The first one retrieves the file size, the second one is an helper function to format the size as a human readable string.

--------

I think this is a basic segment (probably more informative than the file format one, sometimes).
Obviously the segment should be added in the color scheme and in the theme config file for it to be visible.