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
