support for mmif_docloc_ plugins

[keighrim]

(fixed#222)
mmif_docloc_ plugins can provide additional handler code to resolve URI/IRI string from document.location value to a local file path so that ClamsApp can process the data as a regular file.

[codecov]

Codecov Report

Merging #223 (6f50a38) into develop (8aaae35) will decrease coverage by 0.39%.
The diff coverage is n/a.

@@ Coverage Diff @@## develop #223 +/- ##
===========================================
- Coverage 94.61% 94.23% -0.39% 
===========================================
Files 6 6 Lines 799 815 +16 ===========================================
+ Hits 756 768 +12 - Misses 43 47 +4 

Flag	Coverage Δ
unittests	94.23% <ø> (-0.39%)	⬇️

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see 1 file with indirect coverage changes