render_site compatibility with Rmd params.

[zeehio]

This PR provides a solution to #903 and #892 as as a side effect.

@jjallaire I used "output_files" as you suggested. Feel free to suggest changes, although I do not have much more time to devote to this.

Highlights:

• render_site understands the output_files field in the _site.yml. If output_files is missing
  it uses the Rmd file with the same name. Example of output_files field:

output_files:
 flights_domestic.html:
   src: "flights_analysis.Rmd"
   params:
     flight_type: "domestic"
 flights_international.html:
   src: "flights_analysis.Rmd"
   params:
     flight_type: "international"

• The output_files feature can be used as well to set as source for an html file a R script that will be spinned:

output_files:
 flights_domestic.html:
   src: "flights_analysis.R"

• render_site also understands the autospin field in _site.yml. If autospin: true is set, render_site will render all R scripts spinning them first. (closes Use spin with render_site #892)

• Tests have been enhanced a bit to cover these features and documentation has been added as well to the ?render_site man page.

I already signed the contribution agreement when I submitted my previous PR (#647)

Comments, patches and merges are very welcome

[zeehio]

Hi,

Sorry for not coming back earlier, we are all a bit too busy I guess...

I rebased this rather old PR that offers the possibility to render Rmd files with parameters with render_site, as well as the option to autospin source R files into Rmd files.

Would you still be willing to review and merge this?

Thanks!

EDIT
On a second thought, I want to review the PR. It was written long ago...

[zeehio]

There is a breaking change in this PR, as it changes the generator$render function so the input argument can be interpreted as an output file. This may have many consequences, so I prefer to close this PR. The branch is there, so if you are interested in taking some things feel free to do so.

The autospin feature was nice, so I will try to create a new PR for that exclusively