Add --root argument and %!TEX root support

[vmartinv]

This should address #131 following the suggested approach in this comment, that is:

• Leave the basic sub-file behavior as it is now
• Add support for the !TEX root magic comment (Error with inputs and includes #81 (comment)) so that one can check sub-files individually
• Add the --root file.tex command line option that does the same thing
• In the Readme, refer users to the tools mentioned in the SO thread if this behavior is not enough for them

Implementation details

• When using the %!TEX root directive, the path provided should be treated as relative to the current file.
• When using the --root argument, the path provided should be treated relative to the working directory.

Hence the implementation of these two features are a bit different.

Corner case

Due to the current logic if you for example supply both %!TEX root = ../root.tex and --root dir/root.tex options, it might generate incorrect paths: dir/../child.tex. But I don't think this should happen often. I can add additional logic to ignore the %!TEX root if --root is supplied, but is it necessary? There's no need to supply --root if the directives are present.

[vmartinv]

Also, note that this diff moves the path resolving logic inside LatexCleaner, this is because in the future I want the --clean command to also expand the includes, so the cleaner will need to be able to locate the files.

[sylvainhalle]

Thanks, this was one feature I was reluctant to dig into. I'm glad someone did it for me! ;-)