Significant overhaul

1. Config is now an interface, and not a map.
2. Added support for selecting subsets of configs.
3. Value accessors will now panic, rather than
   return errors.
Add tests for Config.Select
Update README to reflect API changes
Restrict boolean values to "on" and "off"
"on"/"off" is stupid

Use "true" and "false" instead, for better compatibility with Go.
Use ioutil.ReadFile()
Enormous overhaul of parse.go

Input parsing has been broken up into three distinct functions, each with
its own set of tests.
Simplify parts of literals.go, make it safer
Add duration overflow tests