Add LaTeX-cwl auto-completion support

* Modify getTeXRoot.py to support relative path in project file in
option of “TEXroot”

* Add LaTeX-cwl auto-completion support.

By default LaTeX-cwl auto-completion will auto turn on when LaTeX-cwl
package is installed.

You can also add configurations as follows in configuration files:
```json
"cwl_list" : ["tex.cwl", "latex-209.cwl", "latex-document.cwl",
"latex-l2tabu.cwl", "latex-mathsymbols.cwl"]
```
which means the cwl list used for LaTeX-cwl auto-completions.