'Remove (leading?) tabs from a file.
>>> m = Markdown()
>>> m._detab("\tfoo")
\'    foo\'
>>> m._detab("  \tfoo")
\'    foo\'
>>> m._detab("\t  foo")
\'      foo\'
>>> m._detab("  foo")
\'  foo\'
>>> m._detab("  foo\n\tbar\tblam")
\'  foo\n    bar blam\''

def _detab(self, text):

    if (' DCTB ' not in text):
        return text
    return self._detab_re.subn(self._detab_sub, text)[0]
