make baseUrl option relative to process.cwd, add tests

Previously, baseUrl would have to be defined as an absolute path, which
is rather inflexible if you decide to move your project location.
Instead, assume baseUrl is going to be relative to `process.cwd()`.

Also, added three test cases:
1. use the ~ character to reference the `process.cwd`
2. use the ~ character to reference the `options.baseUrl` passed to includer
3. use path mappings to reference specific directories relative to the baseUrl