Feat: <Table> header support

[juanesquintero]

Enable <Table/> inner border preview
Add header prop to <Table/>

[juanesquintero]

@wyozi Hey Joonas! Do you think is a good approach to insert a header prop or either way implement a full <Table.Cell> component?

[wyozi]

@juanesquintero if I understood correctly the goal of header prop would be making a cell that is as wide as the table. In that case I think a full-on <Table.Cell> would work more nicely here and be a bit more generic

[wyozi]

Looking at pptxgenjs' TableCellProps type, maybe <Table.Cell> could extend <Text> props too and add the extra props (including colspan). But you can experiment a bit and see what works

[juanesquintero]

@wyozi Agree with you! Will open a PR with that approach...