Use env_logger filters

[IcanDivideBy0]

Hello,
This PR adds env_logger as a dependency to re-use its filter system.
As seen in #21, having the ability to filter modules logs is pretty useful for development. The env_logger crate already does a pretty good job at it, plus controlling this behavior based on environment variables is really useful IMO.
Sadly due to the Logger structs being private to the crate, it's pretty hard to extend the functionalities of femme.
I can understand adding a dependency to env_logger might not be the desired path for this crate, I've tried to add it as a feature without a lot of success. So I see different possibilities:

• use the PR as is and live with the new dependency
• make a public trait and a new function which allows to access the underlying loggers
• publish this pr as its own crate (env_femme?) (having more public things here would be helpful)