Fix table blank header

[peltomaa]

Sorry for the inconvenience, but I was too hesitant to create a PR because the blank header didn't behave correctly in remark-gfm parser I tested.

Changes:

• Table blank header cells are created based on first-row cell count

Generated output:

From:

||
|-|
|**test**|**test**|**test**|
|1234|1234|1234|
|2345|3546|4567|

To:

||||
|-|-|-|
|**test**|**test**|**test**|
|1234|1234|1234|
|2345|3546|4567|

Should I bump the version with every PR?