[RFC] Do not define Markdown.latex using eval
[RFC] Do not define Markdown.latex using eval

This PR solves #356 at the expense of a warning:
```
WARNING: Method definition latex(IO, Markdown.LaTeX) in module Markdown at /home/fredrikb/julia/usr/share/julia/stdlib/v1.5/Markdown/src/IPython/IPython.jl:28 overwritten in module WeaveMarkdown at /home/fredrikb/.julia/dev/Weave/src/WeaveMarkdown/markdown.jl:10.
  ** incremental compilation may be fatally broken for this module **
```
mpastell/Weave.jl