Add test that runs every nannou example listed in the Cargo.toml

[mitchmindtree]

This adds an example that runs each nannou example listed in the
Cargo.toml for a few seconds each, one at a time.

This is useful for finding panic!s that only occur at runtime, as
while cargo test does compile the examples, it does not try and
execute them.

The test has the #[ignore] attribute so that cargo test does not run
the example by default. This is because 1. the test takes a looong time
to run and 2. travis is likely incapable of running a lot of the tests
as it doesn't have a windowing system etc. As a result, to run the test
locally use cargo test -- --ignored. We should try to ensure we run
this at least once before publishing each major release.

Specifically, this test was created to help with testing for vulkano
image layout bugs (#271). cc @freesig just pinging you as you might
find this useful for this reason too!

[mitchmindtree]

Travis seems to be failing due to a bug in the latest nightly (rustc overflowing its stack while building docs):

thread 'rustc' has overflowed its stack
fatal runtime error: stack overflow
error: Could not document `num-traits`.
Caused by:
  process didn't exit successfully: `rustdoc --crate-name num_traits /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/num-traits-0.2.6/src/lib.rs --cap-lints allow --color always -o /home/travis/build/nannou-org/nannou/target/doc --cfg 'feature="default"' --cfg 'feature="std"' -L dependency=/home/travis/build/nannou-org/nannou/target/debug/deps --cfg has_i128` (signal: 6, SIGABRT: process abort signal)
warning: build failed, waiting for other jobs to finish...
error: build failed
The command "cargo doc -v" exited with 101.

[mitchmindtree]

Here is the relevant issue rust-lang/rust#59502

Going to merge this for now as stable and beta seem to be building fine and it looks like there is a PR up already for fixing the issue above in rustc.

[mitchmindtree]

Just a heads up that using cargo test -- --ignored will actually also try to compile and run examples that have been ignored throughout the docs. The correct way to run the new example would be to use:

cargo test run_all_examples -- --ignored