Add cran parameter (#333 and #340 candidate fix)

[maksymiuks]

@gaborcsardi This is a candidate fix for #333

I've realized that fixing the issue reported above is dead simple. In my opinion get_repos() helper should append CRAN to repos only conditionally on some input parameter, rather than every time CRAN is not found in current options. Therefore I've just added this parameter and propagated it upstream including revdep_check() and documentation.

[maksymiuks]

Requesting review @gaborcsardi

[maksymiuks]

@gaborcsardi Is there any chance to address this in the nearest future?

[maksymiuks]

Added candidate fix for #340

[gaborcsardi]

Thanks and sorry for the super long wait!