Fix logic to detect URLs

[simleo]

utils.is_url currently returns False if the netloc field is empty, causing two main problems:

• Failure to recognize mailto: entity IDs as absolute URLs, which results in a leading hash mark being added to the ID
• Incorrect handling of URLs like file:///tmp/foo.txt, which are treated as relative paths

In the latter case, the following call:

crate.add_file("file:///tmp/foo.txt")

Results in an output entry of:

{
    "@id": "foo.txt",
    "@type": "File"
},

Though no file is added to the crate dir (see #75) unless the current working directory has a file:/tmp/foo.txt file in it (file: is a legal directory path in some OSs, e.g. Linux).

With this PR, the generated file entry is:

{
    "@id": "file:///tmp/foo.txt",
    "@type": "File"
},

Note that it's still possible to add a file whose relative (to the CWD) path is file:/tmp/foo.txt (or similar) by passing its absolute path to add_file.