[Feature request] Support for certain TeX directives

[japorized]

Hi there and thanks for creating and maintaining this plugin.

This is more of a feature request and less of an issue. I am hoping that neotex can also support certain TeX directives used in other editors and also vimtex. Here's an example of one of the directives:

% !TEX TS-program = lualatex

On editors / plugins that support this, they will automatically use the specified engine to compile their tex file. This is useful as one may use different engines to render their documents, while neotex currently only allows us to specify one engine. I tried setting the variable g:neotex_pdflatex_alternative while in a buffer that that did not work.

Here are two links for your reference about these directives: TeX SE and TeXWorks Docs

Supporting these directives will also resolve #8 through % !TeX root.