Librarify md-tui

[orhun]

Continuing the work started in #134 (closes #134 & closes #149)

The summary of this PR:

1. Exposed the modules via lib.rs
2. Added a simple demo
3. Gated the networking and highlighting functionality behind feature flags (they are enabled as default)

Now md-tui can be used as follows:

[dependencies]
md-tui = { version = "*", features = ["networking", "highlighting"] }

[henriklovhaug]

Move the changelog into 0.8.5 and I'll merge it

[orhun]

done!

[henriklovhaug]

S&G (smooth & good)