Support for retrieving policies from http urls

[deitch]

First cut at resolving #11

Supports the following:

• file:///path/to/policy/file
• http://host.com/a/b/c
• https://host.com/a/b/c
• path/to/file
• /absolute/path/to/file

The first three are handled by checking that it is a valid URL. The last two are because it treats any invalid URL as a path.

• paths and file: URLs are handled the same way as before
• http and https are handled using bent

What is missing?

• authentication for http/https, although it might be out of scope
• git protocol

My preference would be to ignore git entirely for this cut. Actually, I would prefer to ignore authentication, too, and handle that in a separate PR, unless someone can come up with an easy way to handle it?

[codecov[bot]]

Codecov Report

Merging #12 into master will decrease coverage by 1.56%. The diff coverage is 9.09%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
- Coverage   78.22%   76.65%   -1.57%     
==========================================
  Files          19       19              
  Lines         427      437      +10     
==========================================
+ Hits          334      335       +1     
- Misses         93      102       +9     

Impacted Files	Coverage Δ
safeguards/index.js	73.33% <9.09%> (-6.67%)	:arrow_down:

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[deitch]

I fixed the linting issues. I think this can be reviewed, and we can discuss git URLs and auth in separate PRs.