Add working paths config

An attempt to address timrwood/includer#10

discard explicit token approach (i.e. using some delineator like
{{tokenName}}) and instead just assume that the first path segment pay
be a mapping identifier.  also change the way the baseUrl/project root
is specified in paths, moving from a leading slash approach to using a
`~` character to specify baseUrl/project root.

Ex:
```js
includer('someFileWithIncludes.js', {
paths: {
baz: "~/foo/bar/baz"
}
}, function (err, data) {
// do something with output
});
```
And then in 'someFileWithIncludes.js':
```js
include('baz/someFileInFooBarBaz.js');

// transformed to
include('<project root>/foo/bar/baz/someFileInFooBarBaz.js');
```

Still need to add tests for this.