Special formatting for snippet code

Summary

Here are some special things you can do with code blocks in Cacher.

Highlight specific lines of code - \!h

You can highlight specific lines in both regular and Markdown-based files. To do so, start the line with the prefix \!h.

For instance, the source code for markedlines.md above looks like:

Here are AWS permissions for publicly-readable S3 buckets:

```ruby
{
  "Version": "2008-10-17",
  "Statement": [{
\!h     "Sid": "AllowPublicRead",
    "Effect": "Allow",
    "Principal": { "AWS": "*" },
    "Action": ["s3:GetObject"],
\!h     "Resource": ["arn:aws:s3:::bucket/*" ]
  }]
}
```