Fixes #105.

* src/bin/common_trans.cc (quote_shell_string): Move ...
* src/misc/escape.cc, src/misc/escape.hh (quote_shell_string):
... here.
* wrap/python/spot_impl.i: Wrap escape.hh.
* wrap/python/spot.py: Implement formula.__format__.
* wrap/python/tests/ltlsimple.py: Test it.
* NEWS, doc/org/tut01.org, wrap/python/tests/formulas.ipynb: Document
it.