main: add -v/--version flag

Added a new dependency to take care of pulling in the version from git
at build time.

Args handling is minimalistic, but there wasn't any before, so I figured
this might do for now. (Eventually, some proper `--help` and usage
output might come in handy?)

Signed-off-by: Stephan Renatus <srenatus@chef.io>

src/main.rs

@@ -1,6 +1,7 @@
 extern crate crossterm;
 #[macro_use]
 extern crate downcast;
+extern crate git_version;
 extern crate indexmap;
 #[macro_use]
 extern crate lazy_static;
@@ -11,6 +12,7 @@ extern crate prolog_parser;
 #[macro_use]
 extern crate ref_thread_local;
 
+use git_version::git_version;
 use nix::sys::signal;
 
 mod prolog;
@@ -19,6 +21,7 @@ use crate::prolog::machine::*;
 use crate::prolog::machine::streams::*;
 use crate::prolog::read::*;
 
+use std::env;
 use std::sync::atomic::Ordering;
 
 extern fn handle_sigint(signal: libc::c_int) {
@@ -32,6 +35,11 @@ fn main() {
     let handler = signal::SigHandler::Handler(handle_sigint);
     unsafe { signal::signal(signal::Signal::SIGINT, handler) }.unwrap();
 
+    if env::args().any(|a| a == "-v" || a == "--version") {
+        println!("{:}", git_version!());
+        return;
+    }
+
     let mut wam = Machine::new(readline::input_stream(), Stream::stdout());
     wam.run_top_level();
 }