Added support for colons in section names

This introduces a new parameter, 'key_val_separator', which
can be set in order to override the string that is used
as a separator between the key/value pair of a setting line.
The default is ' = ', but you could set the param to '=' if
you don't want to include whitespace in your settings file.

For now, the key_val_separator is required to contain
exactly one '=' character.  This commit simply validates
that that is the case.

…e-of-separator-str

Allow overriding separator string between key/val pairs