basic URL support

[fonsp]

Fix #7

Some TODOs:

• [ ] Deduplicate downloads & add test
• [ ] Think harder about the filename (for stack traces), right now I append basename(url) to tempname(). We could inject LineNumberNodes with the URL as "filename".
• [ ] More tests?
• [ ] I get a warning about the Downloads stdlib on Julia 1.6

[codecov[bot]]

Codecov Report

Merging #21 (46276d8) into master (13e13d6) will increase coverage by 1.55%. The diff coverage is 100.00%.

:exclamation: Current head 46276d8 differs from pull request most recent head 5b08c05. Consider uploading reports for the commit 5b08c05 to get more accurate results

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   94.87%   96.42%   +1.55%     
==========================================
  Files           1        1              
  Lines          39       28      -11     
==========================================
- Hits           37       27      -10     
+ Misses          2        1       -1     

Impacted Files	Coverage Δ
src/FromFile.jl	96.42% <100.00%> (+1.55%)	:arrow_up:

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[Roger-luo]

I think it's OK to drop 1.5 so we get rid of Downloads warnings, and just support 1.6 as it's the new LTS