add support for converting to em-based units - closes #1

Added a magic variable:

`enable-em-breakpoints`. This is a boolean flag that, when enabled, will use `base-font-size` (if it has been set, otherwise 16px) as the context. I realized we don't need a type conversion library for this because media queries are always relative to the root context (i.e. they behave like rems since there's no nesting going on)

It's also better for this to be off by default, as the user wouldn't expect pixel units to automatically convert to ems.