Add command line option handling

[dcbaker]

Related to #28, but separate. Probably after.

Meson has fairly complex command line handling. Some options can be passed as -Darg or -- arg. Some cannot. Meson's option handling is also undergoing some evolution currently, so that is worth watching to see how they landed. I have no requirement that Meson++ and Meson use the same command line interface, although providing a shim of some kind might be useful.

My current thought is that there are no -- options aside from --help/-h. -Doptname=value sets a global value (a default for all projects) -Dprojectname:optname=value set's a value for a single project, including the root project (possibly with -D:optname=value as a shortcut?` project specific options always have a higher precedent than global values