support for commonmark style links

[tlienart]

indirect links are now supported so stuff like

[link] ... 
[link]: url

or

[link][] ...
[link]: url

or

[link][id]
[id]: url

all are roughly tested but I should probably get a night sleep on it before merging. Probably need to assess a few things:

• does it work with images (I think it should but haven't tried)
• does the Gruber's syntax for link names work in Julia? so like [a link](url "link title") if not, just document it and add support only if explicitly requested.

[codecov]

Codecov Report

Merging #214 into master will decrease coverage by 0.19%.
The diff coverage is 79.31%.

@@            Coverage Diff            @@
##           master     #214     +/-   ##
=========================================
- Coverage      87%   86.81%   -0.2%     
=========================================
  Files          27       28      +1     
  Lines        1131     1160     +29     
=========================================
+ Hits          984     1007     +23     
- Misses        147      153      +6

Impacted Files	Coverage Δ
src/converter/md_utils.jl	100% <ø> (ø)	⬆️
src/manager/file_utils.jl	82.69% <ø> (ø)	⬆️
src/JuDoc.jl	100% <ø> (ø)	⬆️
src/converter/html.jl	95.65% <100%> (+0.19%)	⬆️
src/converter/fixer.jl	78.57% <78.57%> (ø)

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