Feature: Include files and incdirs from the environment.

[DaveMcEwan]

As per #269, thanks @Sv3n.

• 3 new environment variables SVLINT_INCDIRS, SVLINT_PREFILES, and SVLINT_POSTFILES to facilitate always-used files, such as a UVM installation.
• The environment variables are each a colon-separated list of locations on the filesystem, just like PATH, so this mechanism is compatible with Environment Modules.
• This is effectively a shortcut to avoid lots of typing on the command line, so you can use the --dump-filelist option to see exactly what is being passed to the parser.
• In the same way that svlint -Ifoo/ bar.sv only looks for bar.sv in the current directory, but not in the incdir foo/, the files listed in SVLINT_PREFILES and SVLIST_POSTFILES are not searched for in any include directories.
• Minor changes to introduction section of MANUAL to make the PDF easier to navigate (usability of GitHub's rendering is not really changed).

[dalance]

I agree to introducing new environment variables.

[eddygta17]

Including and usage of the UVM library with svlint causes a stack overflow to occur on svlint 0.9.0. Since the merged branch adds only checking environment variables, I have not checked it with the latest codebase.