Support capturing for cargo test

[KodrAus]

Closes #107

Adds an is_test method to the Builder that can be used to force the logger to output using the print or eprint macros instead of through termcolor directly. Using a builder method instead of features guarantees no matter what your env_logger configuration, you'll be able to make cargo test capture logs. is_test is slower than using termcolor.

I've also introduced some convenient crate functions to get a Builder:

• env_logger::builder() is like env_logger::Builder::from_default_env()
• env_logger::from_env(e) is like env_logger::Builder::from_env(e)