Add undocumented `app.openFast()` method

[zlovatt]

Be sure to add the admonition that this is undocumented!

[bjg2]

anyone has any idea how app.open() differs from app.openFast() ?

[Nico-Duduf]

Done, PR #35

I've measured it to be noticeably faster with many small projects (which open in less than 2 seconds anyway). The example in the doc in the PR is a simple script to measure the difference.