support mapping-style % formatting in Markup

[cboos]

Currently, Markup supports % formatting for simple %s, %d, etc. substitution:

    m = Markup("Total %d for group %s") % (123, 'closed')

However, the interpolation with named keys is not supported.

    m = Markup("Total %(total)d for group %(group)s") % {
        total=123, group='closed'}

[cboos]

Adds mapping-style interpolation to Markup constructor and __mod__ operator

[cboos]

Second version of the patch, dropping support for interpolation in the Markup constructor.

[cboos]

The second patch (suggested by cmlenz) is indeed simpler.

Note that Markup_new has been a bit hurried, there's no error checking proper, only truncation of the extra args. We might consider another approach, simply forwarding the all the args wholesale to unicode and have encoding support and error checking for free ;-)

[cboos]

Actually, now that we started down the way to break compatibility on Markup constructor, I think that not implementing __new__ at all is way better. For example, we can now build Markup strings specifying the encoding directly:

>>> Markup("C'est bientôt <b>l'été</b>!")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0x93 in position 11: ordinal not in range(128)

>>> Markup("C'est bientôt <b>l'été</b>!", sys.stdin.encoding)
<Markup u"C'est bient\xf4t <b>l'\xe9t\xe9</b>!">

And it's less code, the diffstat for the patch is 48 insertions(+), 52 deletions(-)!

[cboos]

3^rd attempt, trying out the idea of skipping the Markup __new__ static method altogether.

[cmlenz]

Patch applied in [829]. Thanks!