Disable auto-generated form field IDs by passing nil for "id" attribute

Previously it was not possible to opt out of auto-generated ID values
for various form fields.
Merge pull request #41 from github/disable-generated-id

Disable auto-generated form field IDs by passing nil for "id" attribute
github35
build using 2.1 first
Merge remote-tracking branch 'origin/2-3-github' into journey