Allow TEMPLATE_STRING_IF_INVALID to contain a placeholder for the missing variable's name

Matt,

This sounds reasonable. A few changes to the existing patch(es) I'd like to see:

1. Needs a test or two.
2. Just check if '%s' in TEMPLATE_STRING_IF_INVALID, rather than doing the count() call. If there's more than one format string in that string, it's okay to error out, since that's a developer error that they can easily fix as part of the debugging cycle. So check if '%s' is in the string, then do the substitution if so. Python will raise an error anyway if you include more than one format marker ("not enough arguments for format string"), so you don't need to do anything extra there.
3. You might want to consider setting a module variable to indicate if the format string is present. Settings values shouldn't ever change (it's a bug if they do), so checking it once and storing the result in a flag for subsequent times will be a performance improvement.

If you could make one combined patch for everything when you update the patch, that would be cool. Also consider including a patch for AUTHORS so that I don't have to try and spell your name correctly.

As requested, tests added and suggested modifications made.

Rather use URL than email

For future reference: you should not access settings.* in code that is executed at import time. Doing so means that you cannot import the module in question without haing already configured settings, which requires messy code design in the manually-configured settings case. We should allow people to follow PEP-8 formatted code practices: all imports first, then execute whataver is required (settings configuration, if needed).

I've altered the patch slightly so that settings are only accessed when required (inside Template methods).

(In [5167]) Fixed #3753 -- Allow optional display of invalid variable name in TEMPLATE_STRING_IF_INVALID. Thanks, Matt McClanahan?.