Create a `.gt_table_center` CSS class and use it as a default option

[rich-iannone]

To allow setting a table to be centered in the content area, we could create a .gt_table_center CSS class and set that by default (perhaps with other options to left/right-align the table). It would contain these CSS rules:

margin-left: auto;
margin-right: auto;

[andrewheiss]

Was about to open a new issue, but found this. Currently the only way I've found to left/right align HTML tables is to include this as a separate CSS file:

.gt_table {
  margin-left: 0 !important;
  margin-right: 0 !important;
}

It works, but it'd be nice to have something in tab_options() that could allow for non-CSS overrides.