Catch up reviewing open PRs

[consideRatio]

We are falling behind with our review effort of open PRs, and I'm not comfortable cutting a release before ensuring we have at least made some progress reviewing open PRs waiting for a maintainers review / merge.

Currently open PRs:

• Update submodules and do recursive clone #123
• Optional command-line keyword arguments #192
• Added non-git source puller functionality #194
• Link generator handles non-git sources #195
• Plugin documentation #229
• Support building on mybinder.org to ease testing #258
• Add binder-badge bot #259
• Add leading message to raised errors' #276
• support running .nbgitpuller.script.{init,update} on start #291
• Forward query string when redirecting from /git-pull/ endpoint #303
• Link generator - support generating links for named servers #309