Added LyX editor backup file extension: i,e .lyx~

This reverts commit d878f045.

Source: https://texdoc.net/pkg/xmpincl -> page 3:

> basically create a new file .xmpi

Added a temporary file used in MacTeX

* without . the auto directory is still considered for commits

* small addition that ensures all Emacs-Lisp files (.el files) are ignored

The presentation software pdfpc [1] (which is handy for latex beamer slides) supports additional notes on the secondary screen provided by *.pdfpc files.
These can be autogenerated from latex documents with the pdfpcnotes package [2], thus should be ignored in a latex project (but not in others where they could have been created manually).

[1] https://pdfpc.github.io/
[2] https://github.com/cebe/pdfpc-latex-notes

minitoc also generates .stc, .slf, and .slt files

- .soc - package changes
- .spl - documentclass elsarticle (Elsevier journals)

Source: http://tex.stackexchange.com/a/256803/9075

The `endnotes` package generates `*.ent` files. These should also be
ignored.

Add missing beamer temporary file extension

Added a line that adds the `auto` folder that Emacs+ AUCTex automatically creates in the gitignore list. 

More about this `auto` feature below 
https://www.gnu.org/software/auctex/manual/auctex/Parsing-Files.html

See documentation at
https://github.com/gregorio-project/gregorio

On Windows, when PDF is opened with Acrobat Reader, one gets following output:

    ! I can't write on file `document.pdf'.
    Please type another file name for output:

If one simply presses "Enter" to continue, the file `.pdf` is generated. Since this is a common case and `.pdf` is never used as full file name, this file should be ignored, too.

Add support for easy-todo package (that produces .lod file during the compilation)

When we use 'thmtools' package for creating 'List of Theorems', auxiliary file '.loe' is created, just as '.toc' for 'Table of Contents' and '.lof' for 'List of Figures'.

Latexian adds some additional temporary files, which are not required for building the project. Therefore they are better left out as well

A precompiled header(`latex -ini`) compiled creates a `.fmt` file.
To speed up xypic's drawing, precompiled matrices are created with the `\CompileMatrices` entry in the preamble. This creates `*.xyc` files.

[cprotect](https://www.ctan.org/tex-archive/macros/latex/contrib/cprotect?lang=en) tracks its contents in an auxiliary file, this is safe to ignore.

The pdfcomment package generates *.upa and *.upb files when compiling LaTeX files. These should be ignored as well.

endfloat.sty produces these two auxiliary files.
From Section 3.2 of its documentation:
> Loading it will have LTEX produce two extra files with .ttt and .fff extensions (for tables and figures, respectively).