Enable variables to be set from config file.

DESCRIPTION

Variables set through the --var option can now be set in the yaml config file with the syntax below.

variables:
    var1: value1
    var2: value2

• Closes Issue #24 (closed) .
• Command line setting of a variable through --var takes precedence over config file.
• Uses the same yaml config file as the system. Variable is set as a top-level entry.
• Required modifications to the parse_system_file() of src/swatch/config.py.