Add a feature where a string data formatter can now be partially composed of...

Add a feature where a string data formatter can now be partially composed of Python summary functions
This works similarly to the {thread/frame/process/target.script:...} feature - you write a summary string, part of which is

${var.script:someFuncName}
someFuncName is expected to be declared as
def someFuncName(SBValue,otherArgument) - essentially the same as a summary function

Since . -> [] are the only allowed separators, and % is used for custom formatting, .script: would not be a legitimate symbol anyway, which makes this non-ambiguous

llvm-svn: 220821