freebsd compatibility

[jostreff]

Dear Developers,

please add FreeBSD also as possible build targets of the plugin. I have building myself changing only 2 files in your source - Makefile and plugin.json to make custom build of the plugin for my self hosted mattermost community running on FreeBSD13.

My changes are:

        mkdir -p server/dist;
++        cd server && env GOOS=freebsd GOARCH=amd64 $(GO) build $(GO_BUILD_FLAGS) -o dist/plugin-free
bsd-amd64;
        cd server && env GOOS=linux GOARCH=amd64 $(GO) build $(GO_BUILD_FLAGS) -o dist/plugin-linux-
amd64;

---

    "server": {
        "executables": {
++            "freebsd-amd64": "server/dist/plugin-freebsd-amd64",
            "linux-amd64": "server/dist/plugin-linux-amd64",