Implement the label parameter for the guide lines.

[remotedots]

Add label as an optional parameter for the guide lines. If you find the idea good, I can also update the Quick Reference Guide.

Example

Running guides.extend([guide((20, 20), 180, label='Page margin')]) would output this:

[spakin]

Merged (dbafcf7ce8b54666db5ae51a3f82e02762c616fc). Thanks for the pull request!