Settable font options

[rich-iannone]

This PR adds the ability to specify additional table fonts before the default set in the font family. This is done globally with the opt_font() function, which either accepts a local font name, or, could use the google_font() function (for specifying a webfont hosted on the Google Fonts service).

There is also the new ability to specify a Google Font through google_font() in cell_text()'s font argument.

Here is some testable code:

exibble %>%
  dplyr::select(char, time) %>%
  gt() %>%
  tab_style(
    style = cell_text(font = c("Comic Sans MS", "Menlo", default_fonts())),
    locations = cells_body(columns = vars(time))
  ) %>%
  opt_table_font(font = list(google_font(name = "Dancing Script"), "Menlo"))

Fixes: #595
Fixes: #585
Fixes: #589
Fixes: #598
Fixes: #404

cc @jthomasmock

[jcheng5]

Add unexported function for appending to initial_css. This function would paste(collapse="\n") its CSS argument, and have an allow_duplicates=TRUE/FALSE parameter that determines whether it should be added if it is already %in% initial_css.

[jcheng5]

Notes from in-person discussion.

1. Should this be generalized from an "include arbitrary CSS" feature to "prepend/append arbitrary htmltools HTML"?
2. Do we need a dedicated set of functions for composing CSS from R, and if so, should they live here or htmtools?
3. Do we support scss?
4. How can provided CSS use the id for table-instance-specific rules?