Add table format "colon_grid"

[dn813]

To close #332, this PR adds a new table format "colon_grid" which is the same as the "grid" format but adds colons to the line below the header to indicate a column's alignment. This requires forcing left alignment on the data in the text output of tabulate.

With this format, tabulate can create Pandoc-compatible tables with alignments.

Existing grid format:

+------------+------------+------------+
| Column 1   | Column 2   | Column 3   |
+============+============+============+
| 1          | a          | 1.23       |
+------------+------------+------------+
| 2          | b          | 12.3       |
+------------+------------+------------+
| 3          | c          | 123        |
+------------+------------+------------+

Grid format using existing alignment functionality:

+------------+------------+------------+
|   Column 1 | Column 2   |   Column 3 |
+============+============+============+
|          1 | a          |       1.23 |
+------------+------------+------------+
|          2 | b          |      12.3  |
+------------+------------+------------+
|          3 | c          |     123    |
+------------+------------+------------+

Broken Pandoc rendering:

New colon_grid format:

+------------+------------+------------+
| Column 1   | Column 2   | Column 3   |
+===========:+:===========+===========:+
| 1          | a          | 1.23       |
+------------+------------+------------+
| 2          | b          | 12.3       |
+------------+------------+------------+
| 3          | c          | 123        |
+------------+------------+------------+

Pandoc rendering of a colon_grid:

[astanin]

There is a extra empty line in the colon_grid doctests (tabulate/__init__.py:1865 and 1877).

Also the second doctest fails even when these empty lines are removed (there is no ':' in the first column).

_______________________________ [doctest] tabulate.tabulate _______________________________
1864     ...                ["strings", "numbers"], "colon_grid"))
1865     +-----------+-----------+
1866     | strings   | numbers   |
1867     +:==========+:==========+
1868     | spam      | 41.9999   |
1869     +-----------+-----------+
1870     | eggs      | 451       |
1871     +-----------+-----------+
1872
1873     >>> print(tabulate([["spam", 41.9999], ["eggs", "451.0"]],
Differences (unified diff with -expected +actual):
    @@ -1,5 +1,5 @@
     +-----------+-----------+
     | strings   | numbers   |
    -+==========:+:==========+
    ++===========+:==========+
     | spam      | 41.9999   |
     +-----------+-----------+

...\python-tabulate\tabulate\__init__.py:1873: DocTestFailure

[dn813]

Thanks for pointing out the failing doctests, that's something I should have checked! Should be working now.