Allow users to pass in additional linkopts to pybind_extension (#11)

Co-authored-by: chuckx <chuckx@cold-sun.com>

build_defs.bzl

diff --git a/build_defs.bzl b/build_defs.bzl
index f857776..41147e1 100644
--- a/build_defs.bzl
+++ b/build_defs.bzl
@@ -29,6 +29,7 @@
         name,
         copts = [],
         features = [],
+        linkopts = [],
         tags = [],
         deps = [],
         **kwargs):
@@ -39,7 +40,7 @@
         name = name + ".so",
         copts = copts + PYBIND_COPTS + ["-fvisibility=hidden"],
         features = features + PYBIND_FEATURES,
-        linkopts = select({
+        linkopts = linkopts + select({
             "@pybind11//:darwin": ["-Wl"],
             "//conditions:default": ["-Wl,-Bsymbolic"],
         }),