Tidy up summary and as_gt functions

[LittleBeannie]

The summary and as_gt functions in their current state are extremely lengthy. A preferable approach would be to condense them into a shorter function that utilizes sub-functions where necessary.

[LittleBeannie]

Hi @jdblischak , considering we have couple of PRs to improve as_gt and as_rtf (such as #458, #450, #449), can we close this issue? I can create another issue to tidy-up the nested if-else in summary.gs_design in another issue, to make the issue more specific.

[yihui]

Or I can create a PR to clean up the summary functions, and close this issue once that PR is merged.

[LittleBeannie]

Sounds great, thank you, @yihui !