test-in-docker: quickly test frameworks in docker

This adds `./test-in-docker` for quickly playing with various frameworks.

All the containers are based off Ubuntu 14.04 which has ZSH 5.0.2.

Thanks to @dritter for figuring out all the framework installation
methods.

docker/zgen/zshrc

@@ -0,0 +1,10 @@
+# load zgen
+source ~/.zgen/zgen.zsh
+
+# if the init scipt doesn't exist
+if ! zgen saved; then
+  zgen load ~/p9k/powerlevel9k.zsh-theme
+
+  # generate the init script from plugins above
+  zgen save
+fi