minify HTML templates

[Rich-Harris]

fixes #15. This is separate to #172 — it only minifies app/template.html itself (and only in prod), not the end result with rendered components. Still, it shaves off a few bytes which is nice.

I suppose we could improve on this by doing the minification at build time and writing the template to the build directory — something to consider at the same time as #181.

[Rich-Harris]

I figured it was worth addressing #181 at the same time. This PR adds the following environment variables:

• SAPPER_BASE — defaults to cwd
• SAPPER_APP — defaults to ${SAPPER_BASE}/app
• SAPPER_ROUTES — defaults to ${SAPPER_BASE}/routes

SAPPER_DEST can still be overridden during dev, if necessary (for build and export it's part of the command itself).

Also, minification now happens at build time, which means a faster startup.