web-sys shouldn't use env_logger

[Pauan]

Motivation

When using web-sys, it pulls in the env_logger dependency, which in turn pulls in all of these dependencies:

aho-corasick
atty
humantime
quick-error
regex
regex-syntax
termcolor
termion
thread_local
ucd-util
utf8-ranges
winapi
winapi-util
wincolor

This means that even a simple "hello world" app takes a lot longer to compile.

As far as I can tell, env_logger isn't actually needed, it's solely for debugging. So I propose that it be hidden behind a Cargo --feature, so that way it doesn't affect people who don't need the logging.

[alexcrichton]

An excellent idea!