Fix search highlight for non-unicode chars

The `matching_bytes` function takes a `&Token` now and:
- gets the number of bytes to highlight (unchanged).
- uses `Token.num_graphemes_from_bytes` to get the number of grapheme
  clusters to highlight.

In essence, the `matching_bytes` function returns the number of matching
grapheme clusters instead of bytes. Should this function be renamed
then?

Added proper highlighting in the HTTP UI:
- requires dependency on `unicode-segmentation` to extract grapheme
  clusters from tokens
- `<mark>` tag is put around only the matched part
    - before this change, the entire word was highlighted even if only a
      part of it matched

http-ui/Cargo.toml

@@ -17,6 +17,7 @@ once_cell = "1.5.2"
 rayon = "1.5.0"
 structopt = { version = "0.3.21", default-features = false, features = ["wrap_help"] }
 tempfile = "3.2.0"
+unicode-segmentation = "1.6.0"
 
 # http server
 askama = "0.10.5"