Make flate2 as a optional dependencies/feature, current is only used to read /proc/config.gz

[pymongo]

In many Rust projects which using procfs, mostly not use kernel_config() API

e.g. tikv project not use procfs::kernel_config()

I hope procfs's default feature is:

[features]
default = ["chrono", "kernel_config"]

let user to determine whether enable flate2 dependency

Can I write a PR for it?

[eminence]

Hi,

This is a good idea. Actually I added this feature last month, but forgot to push and make a PR. I just opened the the PR. Can you take a look to see if it looks OK to you?

[pymongo]

resolve in #157