Coercing values into element objects  #26
Text can be automatically casted from string  #26
Mark can be automatically casted from bool

Fix #26
Merge nullable child elements
Properly merge subscription lists
Make crawler to immediately start when it’s called

Previously it starts when it firstly yields
stage.subscriptions is not updated
Merge branch 'master' of https://github.com/earthreader/libearth into bugs