This allows the boolean options to be reconfigured, e.g:
```go
options := getOptions()
options = append(options, html.WithLineNumbers(true))
```
Fixes#301
Since these are wrapped in a `<pre>`, newlines hint the browser that the
line numbers should be on separate lines. This helps when rendering
content with broken CSS, or in a text-only browser.
This was done to speed up incremental compilation when working on
lexers. That is, modifying a single lexer will no longer require
recompiling all lexers.
This is a (slightly) backwards breaking change in that lexers are no
longer exported directly in the lexers package. The registry API is
"aliased" at the old location.
This is to solve an issue where writers returned by the Formatter
were often stateful, but this fact was not obvious to the API consumer,
and failed in interesting ways.
