handle markdown table column alignment

[stefanprobst]

currently, the mdx field will remove any table column alignment on save:

| title | counts |
| ----- | -----: |
| one   | 1      |
| two   | 123    |

will be saved as:

| title | counts |
| ----- | ------ |
| one   | 1      |
| two   | 123    |

(note that the colon : which marks the column alignment is removed)

[tpougy]

Any news on this issue? A fix for this bug would be of great help to my blog since Im trying to use keystatic to manage the content.