Add syntax highlighting to markdown rendering

The feature tries to work as much as possible like GitHub's rendering.
Syntax highlighting is done with Sublime syntax files using syntect.

Also made some changes to the config used with the HTML sanitizer to
be more flexible with the allowed tags & attributes.

Author: kureuil

Cargo.lock

@@ -52,6 +52,7 @@ clippy = { version = "=0.0.142", optional = true }
 chrono = "0.4.0"
 pulldown-cmark = { version = "0.0.15", default-features = false }
 ammonia = "0.5.0"
+syntect = "1.7.1"
 
 conduit = "0.8"
 conduit-conditional-get = "0.8"

Cargo.toml

@@ -37,6 +37,7 @@ extern crate rand;
 extern crate s3;
 extern crate semver;
 extern crate serde;
+extern crate syntect;
 extern crate time;
 extern crate toml;
 extern crate url;