fix #173

New CLI parameter:

--headers HEADERS
  where HEADERS can be "firstrow", "keys", "HEADER1,HEADER2,...", "KEY1:HEADER1,KEY2:HEADER2,..."

--read FILEFORMAT
  where FILEFORMAT can be "rsv" (default), "csv", "jsonl"

Deprecated CLI parameter:

--header

Usage examples:

cat ./examples/people.csv | tabulate -r csv --headers firstrow
  id  name    email              "favorite" fruit
----  ------  -----------------  ------------------
   1  Alice   alice@example.com  apple, kiwi
   2  Bob     bob@example.com    banana,
                                 orange,
                                 lychee
   3  Carol                      pear

cat ./examples/people.jsonl | tabulate -r jsonl -f grid --headers 'id:ID,name:First Name,email:Email'
+------+--------------+-------------------+
|   ID | First Name   | Email             |
+======+==============+===================+
|    1 | Alice        | alice@example.com |
+------+--------------+-------------------+
|    2 | Bob          | bob@example.com   |
+------+--------------+-------------------+