Auto merge of #11262 - campaul:add_version_flag, r=Manishearth

Add `--version` flag

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy --faster` does not report any errors
- [X] These changes fix #11241 (github issue number if applicable).

Either:
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

Not 100% sure of a good way to test this, so I'm submitting as is for feedback. Manually testing it appears to work fine.

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components/servo/main.rs

@@ -40,6 +40,7 @@ use servo::Browser;
 use servo::compositing::windowing::WindowEvent;
 use servo::util::opts::{self, ArgumentParsingResult};
 use servo::util::panicking::initiate_panic_hook;
+use std::process;
 use std::rc::Rc;
 
 pub mod platform {
@@ -102,6 +103,11 @@ fn main() {
         return servo::run_content_process(token)
     }
 
+    if opts::get().is_printing_version {
+        println!("Servo {}{}", env!("CARGO_PKG_VERSION"), env!("GIT_INFO"));
+        process::exit(0);
+    }
+
     let window = app::create_window(None);
 
     // Our wrapper around `Browser` that also implements some