TODO list, most priority at the top.

 * Problem of tarballs which disappear when package is updated
 * Solve issue of depending on a newer version of a base package than what is
   available in the currently installed version of R
 * Do-not-process feature to list packages we shouldn't even look at
 * Allow per-package keywords for:
    - packages which require e.g. 64 bit only
    - packages present in multiple repositories
    - ?
 * Better way to deal with R suggestions
 * Handle slots in fuzzy dependency resolver
 * readline in depres_console