Added the ability to substitute an Eclipse variable into the feature path

Added the ability to substitute an Eclipse variable into the feature path name while creating a "Run Configuration". This helps when sharing launch configurations across team members by using a relative path instead of an absolute one. For example:
Feature Path: `${project_loc}/src/tests/features/belly.feature`

I didn't specify a version for the `org.eclipse.core.variables` plugin because I couldn't seem to get anything other than 3.2 to show up as an available dependency. It looks like there is at least a 3.7 version and it's part of core so I'm not too worried, but if you think this is an issue, I can add a [3.5.0) requirement.

`substituteVar` is stolen from: https://github.com/eclipse/m2e-core/blob/86b43f8aabe312f643fa08fd8320a5649a382939/org.eclipse.m2e.launching/src/org/eclipse/m2e/internal/launch/LaunchingUtils.java#L28