Pre-update commands (fix #25)

README.md

@@ -55,6 +55,14 @@ git_repos = [
     "~/dev/topgrade",
 ]
 
+[pre_commands]
+"Emacs Snapshot" = "rm -rf ~/.emacs.d/elpa.bak && cp -rl ~/.emacs.d/elpa ~/.emacs.d/elpa.bak"
+
 [commands]
 "Python Environment" = "~/dev/.env/bin/pip install -i https://pypi.python.org/simple -U --upgrade-strategy eager jupyter"
 ```
+* `git_repos` - A list of custom Git repositories to pull
+* `pre_commands` - Commands to execute before starting any action. If any command fails, Topgrade
+  will not proceed
+* `commands` - Custom upgrade steps. If any command fails it will be reported in the summary as all
+  upgrade steps are reported, but it will not cause Topgrade to stop.