add a --bootstrap mode for configure.py

Instead of bootstrapping through a separate script, instead make
configure.py able to either generate a build.ninja *or* just execute
all the computed commands to build a ninja binary.

Note: this isn't complete.
- there's one XXX 
- I'm not happy about the sys.exit() bit
- need to replace/update bootstrap.py, docs, etc.


Should that be here then? If it's in this file, it'll eventually grow ~full support. (I suppose it's here because configure.py doesn't have a test suite – maybe just adding a few

    assert _exp('foo', {'x': 'X'}) == 'foo

lines below the definition of the function in configure.py would be good enough though? These tests probably take less than 1ms, so running them on every configure doesn't seem terrible.)

Up to you though.