Add program options for adding include and tool search directories

[StanHash]

Core A FE8 -I:Some/Include/Path -T:Some/Tool/Path [...]

This adds two (four) new commandline options: -I:Path (or --include:Path) and -T:Path (or --tools:Path). The former adds a path for searching included files in, the latter adds a path for searching tools in.

To get this to work, I had to rework some internals a bit:

• I "replaced" the file-searching functionalities of the static IO.IOHelpers with a new non-static IO.IncludeFileSearcher class, which holds a list of include paths and general configuration for the purpose of being able to change all of that at runtime (and have multiple configurations).
• I made Preprocessor.Handler (now DirectiveHandler) non-static so that we are able to construct/configure directives after reading arguments.

Closes #32.

[Crazycolorz5]

Is it possible to have an -IT:path that behaves like -I:path -T:path ?

Also do we wanna rethink the colon syntax while we're here? lol.

[StanHash]

Do you want me to add support for -I <path> stuff (and same for all other options with argument) in this?

[Crazycolorz5]

Maybe in a different issue, if it becomes an issue. : notation is kinda bad and eh, but changing it would break things so nah. It's a rock and a hard place.