Make dependence on rustls optional

[redmie]

I host on constrained resource behind a nginx reverse proxy that handles https.
As such I would benefit from having rustls as an optional dep. for reduced attack surface and smaller binary size.

This can be achieved by using cargo features flags (#[cfg(...)])

[dobefore]

good point! I will give this a try.

[dobefore]

A pr has been made,hope you can give some advice,mainly in main.rs

[redmie]

Given my thoughts in the comment of #13.

Big thanks for your contribution and for the time you allocated to this project!