Feature: Automatic passthru to compiled binary

[ghostsquad]

Description

This is a feature request to have the globally installed mage passthru to the compiled binary file if, for instance an ENV variable is set, or a local config file is specified as such.

Benefits

Make this feel more like make, since folks would not be familiar with calling something like ./make.

Risks

If mage ever wanted to implement subcommands, this would prevent it, as there would be risk that subcommands would shadow local targets.

Example Behavior

Config file like .mageconfig

passthru: ./compiled_mage

or env var

MAGE_BINARY_PASSTHRU=./compiled_mage

mage build would really just be calling ./compiled_mage build

[ghostsquad]

This is related to #289 as, that would enable autocomplete fairly easily without making assumptions about the name of the compiled binary. Additionally autocomplete would be much more performant that the current mage -l since that has to compile to a temporary binary prior to being able to list the targets (or at least render the template).