fixed a Vim error(E907) in Min/Max when comparing v:null to a float
fixed CountE/CountNE/PercentE/PercentNE to support spanning rows/columns
added test cases for all the new formula functions
If a g:table_mode_separator is escaped by a backslash, the table is
aligned ignoring this instance of the g:table_mode_separator, but the
column deletion and insertion functions did not consider this behaviour.
The g:table_mode_escaped_separator regex is simplified and optimized a
little bit. Also more care is taken to prevent unexpected behaviour if
special characters are used as g:table_mode_separator.
We now have support for center alignment, simply add a header border
with the `g:table_mode_align_char` as both the first and the last
character so it would look something like `+:.....:+` to center align
the column values.
Altered table mode behavior such that if there is a header line, i.e.
the first line of the table is separated by a border with the rest of
the table, then it is not taken into account for formulas and cell
arithmetic
