Reusable environment variables across tasks.

[jespertheend]

The user should be able to configure environment variables for a task and these variables should be replaced when using values in the configuration like $VARIABLE.
When a task runs another task, this child task should inherit the variables set by the parent.
This is the case when running dependency tasks (#9), but it should also be possible for a task to easily run another task programmatically and pass along these variables.
Variables provided by the child should take precedence over the variables from the parent.