Support speedy mode

[chanan]

From glamor readme:

there are two methods by which the library adds styles to the document -

• by appending css 'rules' to a browser backed stylesheet. This is really fast, but has the disadvantage of making the styles uneditable in the devtools sidebar.
• by appending text nodes to a style tag. This is fairly slow, but doesn't have the editing drawback.

as a compromise, we enable the former 'speedy' mode NODE_ENV=production, and disable it otherwise. You can manually toggle this with the speedy() function.