support config of multiple rails via ENV or TOML

factors configuration reading into separate file within same package

uses TOML array syntax to capture rails

environment variable names are noted with zero-based array index (i.e.
GL_RAIL_0_NAME, GL_RAIL_1_NAME, etc.)

updated README to include usage
Merge pull request #5 from abargnesi/configuration_options

support config of multiple rails via ENV or TOML