Allow appPath to be passed as an environment variable

[twolfson]

We are using gm for image-diff, a wrapper around ImageMagick's compare functionality. We recently got an issue requesting support for appPath being passed through from our library to gm itself (uber-archive/image-diff#51).

We are open to supporting this via JS but it seems like something that could be controlled via environment variables (e.g. in Karma, we can pass browser executable paths via _BIN environmnet variables -- e.g. PHANTOMJS_BIN, FIREFOX_BIN).

We were wondering if we could add support for appPath being an environment variable (e.g. GM_APP_PATH, or maybe something less confusing so we have 1 for ImageMagick and 1 for GraphicsMagick)