If you don't mind wasting a line by leaving it empty, consider the following hack (it is a hack, and use this only if you don't like adding any additional plugins).
| | | |
|-|-|-|
|__Bold Key__| Value1 |
| Normal Key | Value2 |
To view how the above one could look, copy the above and visit https://stackedit.io/editor
It worked with GitLab/GitHub's Markdown implementations.