Add ability to set cell-level meta for the header

[djm]

This will allow cell-level alignment in the header cells, which is only currently possible by passing in already formed %Cell{} structs to Table.set_header/2.

Proposed API:

alias TableRex.Table
row = ["Konflict", "Cyanide", "Renegade Hardware", 1999]
header = ["Artist", "Track", "Label", "Year"]
Table.new
|> Table.add_rows(rows)
|> Table.set_header(header)
|> Table.set_header_meta(0, align: :left)
|> Table.set_header_meta(2, align: :right)
|> Table.render!
|> IO.puts
+-----------------+---------------+-------------------+------+
| Artist          |     Track     |             Label | Year |
+-----------------+---------------+-------------------+------+
|    Konflict     |    Cyanide    | Renegade Hardware | 1999 |
+-----------------+---------------+-------------------+------+

This example looks funky but there are cases where the headers need to be aligned separately from the column data below.