Removing Makie, CairoMakie and Plots as requirements via Requires.jl

[davibarreira]

This PR moves some code around in order to improve the organization overall. It also fixes some small bugs:

• It recognizes the text/plain output in Jupyter cells;
• It does not return the subsubsubsection, which does not exists. Instead, it just uses \textbf.

The main change comes from the fact that now it uses Requires.jl in order to avoid Makie, CairoMakie and Plots as dependencies. Theses packages are only necessary for the Pluto converter. Hence, if you just wish to convert Jupyter notebooks, the package precompilation time will be quite low.

[codecov]

Codecov Report

Merging #8 (ed36262) into master (8e25d91) will increase coverage by 0.05%.
The diff coverage is 98.91%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   96.81%   96.86%   +0.05%     
==========================================
  Files           5        7       +2     
  Lines         408      415       +7     
==========================================
+ Hits          395      402       +7     
  Misses         13       13              

Impacted Files	Coverage Δ
src/markdowntolatex.jl	99.17% <0.00%> (ø)
src/plutoconverter.jl	99.13% <99.13%> (ø)
src/NotebookToLaTeX.jl	90.90% <100.00%> (-8.00%)	⬇️
src/jupyterconverter.jl	100.00% <100.00%> (ø)

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