Allow table marker row to have more columns than header

[christianselig]

To match with Reddit's more lax table header/marker syntax (the header being the first row, and the marker row being the row after it that dictates the alignments), this change allows more marker/alignment columns than header columns.

For whatever reason this behavior/syntax is very common on Reddit, I assume because some folks will just edit the header row to remove a column, and then simply not update the alignment/marker row.

For instance this syntax renders a table on Reddit with 3 columns, but does not render with cmark-gfm as it has one too many marker/alignment columns:

markdown

|Header1|Header2|Header3|
|-------|-------|-------|-------|
| Body1 | Body2 | Body 3|

But does render on Reddit as if the last marker/alignment column wasn't present, thus looking like this:

Header1	Header2	Header3
Body1	Body2	Body 3

This PR changes the rules of the table extension to allow more marker/alignment columns than header columns.

It's a seemingly harmless change and all tests still pass. Note that the opposite (more header columns than marker/alignment columns) would fail the tests understandably as some header columns would not have an explicit alignment set.

I totally understand if this isn't behavior you're interested in/care about, I just integrated it into my app and appreciate this library and thus wanted to do the good thing and offer up my work in case you were interested!

(Shoutout to @QuietMisdreavus for helping me, any nice parts of this PR are thanks to her, and any mistakes are totally my fault.)