Don't build what is broken

Previously our meta checks where broken, which lead to packages marked as broken to be built.

Now with comments

There is a new subcommand that will print the latest comments/reviews on
a pr in the console:

If you are in a review shell:

$ nix-review pr 17

You can get the comments with:

$ nix-review comments

This assumes that you have a github token set.

Minor fixes

• fix local evaluation when only packages are removed
• use attribute path when printing diffs between master and PR

Improved GitHub support

Nixpkgs-review can now merge and approve pull requests (See
merge and approve subcommand).
It can also post review results as comments on the pull request
(See post-result subcommand).

Github++

Added post-result/merge/approve subcommands

Github comments

Thanks to @bhipple nixpkgs-review can now post its report to the corresponding github issue.

Fixing evaluation of disabled python packages

• now print more update stats when locally evaluating
• fix evaluation of disabled python packages

Project rename

• rename to nixpkgs-review from nix-review
• switch to relaxed sandbox: d2e3fba
• no longer override nix's max-jobs option: 2bbdf97

Nitpicks

• Escape attributes before writing build.nix
• create build directory under $XDG_CACHE_HOME
• disable pruning in git fetch to work around git limitation
• also start nix-shell if no packages have been changed
• add --remote option to support nixpkgs forks in nix-review rev

"Feature parity"

This releases introduces the wip subcommand from nox-review, thanks to @Ma27.
Now nix-review is at least on par with all features that nox-review provides.