Fix support for separating lines

[pjkundert]

Resolves #246

[davidlatte]

It would be great to get this PR merged and released!

[astanin]

I tried merging this commit with master (see pr-279 branch), but the tests fail in test_simple_headerless_with_sep_line_with_linebetweenrows_in_tablefmt.

Expected output:

+------+----------+
| spam |  41.9999 |
+------+----------+
+------+----------+
| eggs | 451      |
+------+----------+

actual output:

+------+----------+
| spam |  41.9999 |
+------+----------+
||
+------+----------+
| eggs | 451      |
+------+----------+

Possible interaction with the changes introduced in pull request #331.
Or I might have broken it during the merge.

[pjkundert]

A check for SEPARATING_LINE got skipped, @astatin; I added it back in. Passes tests now.

[astanin]

@pjkundert Thank you. I think this can be merged now.