Allow merging table cells vertically using ^^

[raducoravu]

Is your feature request related to a problem? Please describe.
It would be great if the table extension could also support merging table cells vertically.

Describe the solution you'd like
Use "^^" in table cells to state that they should be merge vertically to the cell above.

Describe alternatives you've considered
None yet.

Additional context
Add any other context or screenshots about the feature request here.
https://www.npmjs.com/package/markdown-it-multimd-table