Add config reader

[daniel-v]

Sass builder should be configurable via a config file.

In the config file, one is to be able to

• provide file globs to execute on
• provide compilation strategy (currently dart and binary)
• optionally provide extra arguments for binary compilation strategy

Example 1:

sass:
  builder: dart
files:
- web/*.scss

Example 2:

sass:
  builder: binary
  arguments:
  - node-sass
  - someotherarg
files:
- web/*.scss

#4 depends on this.