Add project_version macro

[stellartux]

This commit implements @project_version, a macro which reads the version number from a Project.toml file for use with the version argument of the ArgParseSettings function. This keeps the version returned by ArgParse in sync with the project's version automatically.

[carlobaldassi]

Looks good, thanks. It should be mentioned in the documentation too though.

[stellartux]

I'd added the docstring to the Settings page. Should I make a separate page for it or write more documentation?

[carlobaldassi]

Ah right I had missed it, sorry.

[GunnarFarneback]

Why is there a Base.current_project() default? This could be absolutely anything, depending on where the current directory happens to be, which is not necessarily aligned with the package code. It could also be nothing and causing a cryptic error, if the current directory (and its parents) doesn't have a Project.toml, which is the case when running automerge checks for the General registry.

It would seem better to just give an error if no file is provided, but I guess that would be a breaking change at this point. Or have a default that actually does make something sensible, but I'm not sure if that's possible (short of insane hacks to try to deduce the caller from the backtrace).

[GunnarFarneback]

Actually I don't figure how the macro could be used in a good way with arguments either, see #136.