Add slim support
Refactor template rendering code

- Create classes for each type of code example
- Use ERB for template rendering

Signed-off-by: Bebe Peng <bpeng@pivotal.io>
Move code example templates into separate erb files

- Rename "code_(example|table)_template" to
"markup_(example|table)_template"
Add ability to create custom code example templates

- Document using code example templates
- Add template files for code example templates