Add feature flag such as `time`,`sha1`

[lexoooooo]

Users don't always need all the functionality, we should use feature flags to let them choose, thereby reducing compilation time

[lexoooooo]

Because time crate, the project relies on winapi and libc, severely affecting compilation time

[seanmonstar]

I have thought that perhaps classes of headers could be optional features. Like auth, conditionals, and the like...

[lexoooooo]

Can I open a pull request?

[seanmonstar]

It'd be best to plan what the features would be first, so that way the code part takes less back and forth. Does my comment about features as groups of headers make sense? Do you have thoughts on logical groupings?

[lexoooooo]

Oh...I don't understand what you mean by groups of headers and logical groupings ...