Fix wrong interface key mappings in examples
Add the document of textobj#user#map
Add tests on textobj#user#map()
Add plain s:interface_mapping_name
Add plain s:normalize
Export textobj#user#map() as a public API
Define failsafe key mappings by textobj#user#map()
Merge branch 'export-map-api'

Close gh-12.
Close gh-14.
Add release notes on 0.5.0
Bump up version to 0.5.0
Merge branch 'prepare-for-0.5.0'