Allow undo for `push` operations
Fix js slate.screenCount
Merge pull request #248 from sabberworm/undo-push

Allow undo for `push` operations
add all move based operations to undo
[#240] add links to about
[#250] load default config properly
[#233] allow numbers for options of move ops
[#241] order of operations =P
[#252] allow integers in screenForRef
[#234] add note to readme