Consolidate replacements into one alist

Consolidate replacements into one alist

New list is which-key-replacement-alist. See docstring.

The following lists are deprecated. Some basic backwards compatibility is
attempted, but more complicated configs will likely break.

which-key-key-replacement-alist
which-key-key-based-description-replacement-alist
which-key-description-replacement-alist

which-key-binding-filter-function was removed, since it's functionality is
mostly replaced by which-key-replacement-alist

Updated README