Adding new templates :article and :matharticle

[davibarreira]

Adding the templates :article and :matharticle, together with update to the documentation and new tests.

[codecov]

Codecov Report

Merging #5 (ce2d357) into master (67503e4) will decrease coverage by 0.14%.
The diff coverage is 97.29%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
- Coverage   96.96%   96.81%   -0.15%     
==========================================
  Files           5        5              
  Lines         395      408      +13     
==========================================
+ Hits          383      395      +12     
- Misses         12       13       +1     

Impacted Files	Coverage Δ
src/markdowntolatex.jl	99.17% <92.85%> (-0.83%)	⬇️
src/NotebookToLaTeX.jl	98.90% <100.00%> (ø)
src/templates.jl	100.00% <100.00%> (ø)

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