Allow default attribute on all fields

When preparing field prompt kwargs, all fields are checked for a
default attribute. An explicitly defined default overrides the first
option in list default. Special username 'submitter' default is still
respected.

Signed-off-by: Aaron Browne <aaron0browne@gmail.com>
Stop generating unique png files on form update

Instead of generating a unique png file when a form is updated,
overwrite the existing png file. This aligns with the way other file
formats are handled. Like other formats, the previous png files are
available in the tracking repo commit history and links to previous png
files remain valid because they are generated using commit sha absolute
link syntax.

Signed-off-by: Aaron Browne <aaron0browne@gmail.com>
Add meaningful README

A much expanded README is added including the primary use case,
requirements, installation, configuration, and usage instructions, and
discussions of template format and field types and attributes.

Signed-off-by: Aaron Browne <aaron0browne@gmail.com>