Can we remove gl as even an optional dependency?

[tmcw]

gl is a big module that's a binary build - it slows down npm install of things that rely on mapbox-gl-js significantly. Could we avoid having it be npm installed when people install gl-js as a dep?

[jfirebaugh]

We can move it to devDependencies.

[lucaswoj]

the proposed optionalDevDependencies feature would have been an ideal solution to this npm/npm#3870 (but it doesn't exist yet)