Add push mechanism which changes ctx only in a subtree
Describe push function in README
Transfer push/pop handling from twalk-dig to twalk-1
Add push* function

Push*-ed change is reverted after :post function.
i.e., you can work on a subtree with a temoporalily changed context.
Bump version to 0.2.0-SNAPSHOT
Merge branch 'feature/push' into develop