{{input}} helper with bindings are now recognized in child helpers

Passing bindings to helpers {{inputField}}, {{hintField}} and
{{labelField}} work fine. Hoerver, it is does not when they are passed
to {{input}} helper. Those commits solve this issue. prompt and
placeholder can be passed as bindings for inputField. Tests have been
added and they pass. README has been modified as well.

This reverts commit f5f2be5984eff3a22c67c9600fffcd76384d002a.