Configurable MML

The previous mml file was hardcoded with datasource information specific to one person's machine's database. This PR adds a small configurable ini file which is used to fill out the relevant bits of a templated mml file. This does add the annoyance of anyone changing the mml has to port their changes (sans the datasource configs) back into the template though so maybe this is sub optimal. does anyone else have a suggestion of a cleaner way of doing this?
No one who I've spoken to actually edits the files in TileMill for any large project, they use an external text editor. I'd just say "Make changes to project.mml"