Allow user-defined tags from external packages, have a way to load an external package in `roxygenize()`

[hadley]

Before:

Roxygen: { library(roxygenlabs); list(markdown = TRUE,
    roclets = c("collate", "namespace", "roxygenlabs_rd")) }

After:

Roxygen: list(
      markdown = TRUE,
      roclets = c("collate", "namespace", "roxygenlabs::roxygenlabs_rd")
    )

[hadley]

This already seems to work, as far I can tell.

[gaborcsardi]

Wow.

[gaborcsardi]

Actually, one issue is defining new tags without defining new roclets. E.g. roxygenlabs has

roxy_tag_parse.roxy_tag_examplesIf <- function(x) {
   ...
}

roxy_tag_rd.roxy_tag_examplesIf <- function(x, base_path, env) {
  rd_section("examples", x$val)
}

which adds a new @examplesIf tag. But this only works if the roxygenlabs package is loaded when roxygenize() is called. So it would be nice if we had a way for loading it at the beginning of roxygenize(). E.g. processx currently has this in DESCRIPTION as a workaround:

Roxygen: { library(roxygenlabs); list(markdown = TRUE) }

Which is not terrible, but it would be cleaner to write

Roxygen: list(markdown = TRUE, roxy_packages = "roxygenlabs")

[gaborcsardi]

One solution would be to load roxy_packages when the options are loaded, at the beginning of roxygenize(). And we would give a warning if they are not available.