This allows the boolean options to be reconfigured, e.g:
```go
options := getOptions()
options = append(options, html.WithLineNumbers(true))
```
Fixes#301
basic version without any options. colors and font-styles seem to be ok.
rough support for text background in styles like `murphy` using predrawn
rectangles (svg has no text background attribute).
things to improve:
- svg width attribute (`<svg width=""`)
- linenumbers
- highlighting
- embedded font
- tabwidth option
- margins?
- better position/width calculation (rectangles not correctly drawn on
resize)
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.
