Feature Request: Allow modifying the caption after creating a gt()

[billdenney]

Prework

• Read and abide by gt's code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Proposal

tab_caption(data, caption) to allow modifying a caption on an already-created gt() object.

data %>%
  gt(caption="old caption") %>%
  tab_caption("my caption here")

As an addition, it would be helpful to also be able to extract the caption information. This may be too flexible, but I would find it helpful to use tab_caption(my_gt) to get the current caption.

[billdenney]

Related to #328

[rich-iannone]

Sounds like a good feature. Thanks for creating this issue!