M@clean_buffer no longer escapes sequences (caused undesirable
behaviour).
Added M@get_selection which will return four values indicating the
starting row/column and ending row/column of a visual-mode selection. If
no selection has been performed in buffer instance, values may be -1.
Value is "cached" as defined Vim behaviour of '< and '> positions.
Added M@subslice is used to get a substring of a word between two
whitespace characters (i.e., spaces). The `line` is the string to check,
and `col` is a position (e.g., from the cursor) within the word which
will be extracted. Return value is a table of starting and ending
indices of the original string.
