Support extra-verbose builds:

- The bootstrap crate currently passes -v to Cargo if itself invoked
with -vv.  But Cargo supports -vv (to show build script output), so make
bootstrap pass that if itself invoked with -vvv.  (More specifically,
pass N '-v's to Cargo if invoked with N+1 of them.)

- bootstrap.py currently tries to pass on up to two '-v's to cargo when
building bootstrap, but incorrectly ('-v' is marked as 'store_true', so
argparse stores either False or True, ignoring multiple '-v's).  Fix
this, allow passing any number of '-v's, and make it consistent with
bootstrap's invocation of Cargo (i.e. subtract one from the number of
'-v's).

- Also improve bootstrap.py's config.toml 'parsing' to support arbitrary
verbosity levels, + allow command line to override it.

src/bootstrap/builder.rs

@@ -763,7 +763,7 @@ impl<'a> Builder<'a> {
             cargo.env("WINAPI_NO_BUNDLED_LIBRARIES", "1");
         }
 
-        if self.is_very_verbose() {
+        for _ in 1..self.verbosity {
             cargo.arg("-v");
         }