validate URIs/URLs

[MartijnR]

external data, media resources

The spec info is here: http://getodk.github.io/xforms-spec/#uris but in addition we consider http(s):// URLs valid too.

[MartijnR]

See Develop for guidance on running this tool locally.

Here is an XForm (extension changed to txt to work around GitHub restriction) that can be used during development (and to add a test spec) that contains several valid and invalid URLs:

1. external <instance> data references i.e. <instance src="....">, always as children of <model>, and
2. image resources i.e. <value form="image">, always under model/itext/translation/text/value as textContent

Create a PR that:

• adds a test spec (optional)
• adds the feature

If you get stuck trying to 'find' the media resources, don't worry about it (and don't spend too much time) and just do the <instance> elements with a src attribute only.