Allow more protocols in URL fields.

[bfabio]

Allow repositories with ssh:// (and its short git form),
git://, svn://, svn+ssh:// as protocol in URL fields, via
a new "repourl" field type.

Also, use https:// as the placeholder for URL fields, to
drop a hint to the user about the nature of such fields.

[libremente]

@bfabio you are right, the publiccode.yml says that the protocols you mentioned could be entered. However, when checking that URL I believe the crawler makes some HTTP GETs but I need to double check that code and get back to you. cc @sebbalex

[bfabio]

@libremente I think you're right: crawler/crawler/crawler.go#L268