Don't activate default features of env_logger

[badboy]

env_logger comes with several optional-but-default-activated features,
including a dependency on regex.
There's no need for android_logger to require that. Deactivating default
features allows users of android_logger to define the features they
need.

We're using regex in our own library, but with [recent changes](https://github.com/rust-lang/regex/pull/613) we were able to reduce our library size by 30% or so. However, now upgrading to the latest android_logger pulls in the whole of regex again, without a way to prevent that. If android_logger just won't ask for the default features our own library is free to specify the final feature set (of course only as long as no _other_ library also pulls in regex with default features).

[Nercury]

Thanks! This looks like a sane thing to do if it reduces the binary size.

This dependency was probably brought in by fancier log filter. I need to check if it works as expected with this change, add a feature to disable it if it does not work for some reason.

[Nercury]

Since this breaks backwards compatibility, would make this crate work differently than default env_logger, would need a new major version and at least a note in readme, let's do it this way:

1. Expose a feature for this crate named regex, that will forward regex to env_logger's regex:

[features]
default = ["regex"]
regex = ["env_logger/regex"]

2. Remove the current change that adds line default-features = false.
3. Add default-features = false for this crate in your project.

[Nercury]

Sorry, looks like we need to leave default-features = false on dependencies.env_logger for this scheme to work, otherwise the regex will still get picked up.

[badboy]

Sorry, looks like we need to leave default-features = false on dependencies.env_logger for this scheme to work, otherwise the regex will still get picked up.

Oh, true. I should not send PRs before my first coffee in the morning :D

[badboy]

These are the default features of env_logger:

default = ["termcolor", "atty", "humantime", "regex"]

So default-features=false and enabling regex will still be different from that.
Hmpf. I'll spend a bit to think about that. Maybe the solution here is something else (and for now I'm fine with depending on an old version of android_logger and in the future I might even replace it due to some other internal code)

[Nercury]

This change is good overall, because we certainly don't need other env_logger features such as colored terminal output or humantime. The only thing we re-use is the filter.

[badboy]

Well, then...