LaTeX: Add font scaling option to package

[abhabongse]

Rationale: When fonts in this package is used in conjunction with other Latin fonts, there may be discrepancies in font sizes. If we can scale the size of fonts in this package, such issue would be solved. This idea is inspired by many font packages such as opensans and roboto, etc.

Changes:

• Font definition files (*.fd) are modified so that they can be scaled by a factor as specified by the command \fontstlwg@scale if it is defined. The syntax used is according to the documentation fntguide.
• The LaTeX package fonts-tlwg.sty is modified so that the command \fontstlwg@scale can be set through package option scaled. It is documented in the file README.latex.
• The package xkeyval is used to allow this package to accept a value for the scaling factor, hence why \DeclareOptionX is used instead of \DeclareOption.
• Tests are added to demonstrate how the scaling option can be used.

[thep]

Thank you for your work! Before I try it, I think the "scaled" option should be "scale" (without "d") to be more intuitive. It's the common option between opensans ("scale") and roboto ("scaled" or "scale"). And users who are familiar with \includegraphics command should be already familiar with its "scale" option.

What do you think?

[thep]

Pull request merged, with option 'scaled' renamed to 'scale'. Thank you very much for your contribution!

[abhabongse]

I agree that scale makes more sense than scaled.