Add support for GLSL shaders

[leakec]

• Similar to how we handle OBJ/MAT files, save contents to a variable.
• When a user specifies a material, check if it is a ShaderMaterial, and if so, check arguments for a file.
• If arguments are files, then load in the variables like we do for OBJ/MAT.

[leakec]

Useful links on GLSL shaders in ThreeJS:

• Here are built-in uniforms/attributes.
• Here is an example YouTube tutorial of GLSL in ThreeJS.

[leakec]

• Support for this has been added.
• Tested in parser_test.
• Technically, ShaderMaterial takes in a set of parameters as its function argument. However, this is different than what we do for other functions where we take in the arguments directly. Here, we force them to be keyword arguments (specified as elements on ARGS rather than as a list), which is also different. I think that this solution is a good one, but it will be useful to document this. Adding as a to-do in #7.