List of filter functions for `filter-buffer-substring'.
Each function must accept a single argument, a string, and return
a string. The buffer substring is passed to the first function
in the list, and the return value of each function is passed to
the next. The final result (if `buffer-substring-filters' is
nil, this is the unfiltered buffer-substring) is passed to the
first function on `filter-buffer-substring-functions'.
As a special convention, point is set to the start of the buffer text
being operated on (i.e., the first argument of `filter-buffer-substring') before these functions are called.