Update to Rust 2018 and make encoding an optional feature

[AlexApps99]

Note: this PR supersedes #17 and #13

This PR:

• Creates an optional (enabled by default) encoder feature
• Updates the crate to Rust 2018
• Removes rand from dependencies and into dev-dependencies
• Fixes two typos made within the encoding section (including one Fix typo in VorbisQuality enum #13 failed to correct)
• Updates all crates to most recent versions, vorbis-encoder is updated to Git master as crates.io version has outdated vorbis-sys

If needed, I can also apply cargo fmt before merging this PR.

This closes #16

[AlexApps99]

@tomaka Are there any modifications I should apply to this PR?

[Hossein-Noroozpour]

Hi it seems good. Thanks.

[AlexApps99]

@tomaka apologies for pinging you again, but it's been a few months, wondering if you're ok with this PR?

[roderickvd]

@tomaka are you still maintaining this repository?

@Hossein-Noroozpour could you update vorbis-encoder and even better, create a public repository for it? It currently is in dependency hell over vorbis-sys.

@AlexApps99 see further and partly conflicting work in #20, I propose that if this is no longer maintained then someone forks it. At this point this crate simply doesn't work on newer Rust versions.

[AlexApps99]

I lost interest in this crate and wrote my own a while ago (don't think I uploaded it), feel free to fork if you want.
I'll close this PR in the mean time.

[roderickvd]

That's a pity. Regardless of your personal interest, your work is sound and should be merged.

[AlexApps99]

If you wish to make a fork, feel free to use whatever is in this PR. Sadly these crates seem to be abandoned by tomaka