Generate vertex tangents for OBJ files

Currently, `rendy::mesh::obj::load_from_obj()` does not generate vertex tangents when loading OBJ files. This prevents OBJ files from being used with Amethyst's RenderBundle's RenderPbr3D plugin, which needs vertex tangents.

I can implement this, but I want to determine if this is a desired feature, and if so, what tradeoffs are preferred before I start. There are 3 ways I see to implement this:

1. Just generate the tangents all the time
* \+ Just works
* \- Wastes CPU cycles if tangents aren't needed
2. Add a boolean parameter to specify if tangents are desired
* \- Breaks API
* \- Inflexible; tangents treated as more special than other vertex attributes
3. Make `load_from_obj` generic over a new trait and implement the trait for `Vec where A: AsVertex` and tuples of those types
* \+ Leaves room for future extension
* \- Breaks API
* \- Adds type complexity

My intuition is that 1 is the best option if OBJ loading time is considered negligible, and 3 is the best option otherwise.