feature: Adds support for RUNFILE env variable

[TekWizely]

Per discussion on #39 this PR adds support for $RUNFILE variable pointing to a runfile.

Behaves like run -r "${RUNFILE:-Runfile}"

As with previous logic, can be overridden with explicit -r|--runfile

TBD:

• Show how it helps reduce verbosity when running tasks from other tasks
• Show how direnv can be used to auto-set the RUNFILE variable when you cd into a folder containing a runfile

---

cc: @rburchell