Syntax highlighting (#691)

* Syntax highlighting mostly... Exists.

* Add dependency to dockerfile

* Handle non-existent languages better

* Make the default a bit nicer

* Improve highlighting. Clean up function

* Add dark theme, add the comment scope to the allowed classes

* update build env

* Address review comments

* Use find_syntax_by_token which produces the desired behavior

* Change flat_map into flatten
(commit cargo.lock)

plume-common/Cargo.toml

@@ -19,6 +19,7 @@ serde = "1.0"
 serde_derive = "1.0"
 serde_json = "1.0"
 shrinkwraprs = "0.2.1"
+syntect = "3.3"
 tokio = "0.1.22"
 
 [dependencies.chrono]