[FIX] - Footnote references

[csantos1113]

This fixes #295.

replaced (.*) with ([^\]]+) , which matches all characters that aren't ]
.
Using options.slugify for the href and id to make sure the footnotes and their references has valid html characters.

[codecov]

Codecov Report

Merging #304 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #304   +/-   ##
=======================================
  Coverage   99.22%   99.22%           
=======================================
  Files           1        1           
  Lines         387      387           
  Branches       65       65           
=======================================
  Hits          384      384           
  Misses          3        3           

Impacted Files	Coverage Δ
index.js	99.22% <100.00%> (ø)

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[csantos1113]

@probablyup could you pls get this merged when possible? Thanks

[quantizor]

Looks great, thank you!