watchMode for @imported stylesheets

[samcday]

Hi there,

First up thank you for developing the awesomeness that is less.js, absolutely love it!

Here is a patch that enables @import stylesheets to be monitored when watchMode is enabled in development environment. When an imported stylesheet changes, those changes are saved and the stylesheet attached to the document is regenerated.

Note that to achieve this I made the decision to keep references to the tree.Parser instances involved in the stylesheets, I figure this should be fine for development mode (there's no memory leaks as far as I can tell).

Thank you.

[cloudhead]

When developing, you can use watch-mode in the browser, by adding #!watch to the url, I don't want this as part of the command line tool.