Add equivalent to ggplot::theme_set() so that desired table options can be specified once for entire document

[kelliemac]

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

There are lots of great table formatting options in tab_options() and it would be really helpful to be able to apply a selection of these to all tables being generated in an Rmd/qmd document, because it is tedious to apply tab_options() repeatedly but this is sometimes needed for consistency of tables across a (potentially long) document.

This would be similar to how ggplot has theme_set() (https://ggplot2.tidyverse.org/reference/theme_get.html) which can be called once at the beginning of a document and then automatically sets the default style for all subsequent figures without the need for any additional function calls.