Support multi-file documents without relying on inline flags

[clamydo]

Please consider supporting multi-file documents without magic editor-specific inline flags inside the document itself. This mixes platform agnostic documents with platform specific build instructions. For good style, IMHO, this should be separated.

A straightforward solution would be to search for a special file, like main.tex.latextools going up the file hierarchy. This file would mark the main.tex file as the root document, that should be compiled.