feat(gopass): add 3rd party fallback completion loader

https://github.com/gopasspw/gopass/blob/master/docs/setup.md#enable-bash-auto-completion

Author: scop

completions/.gitignore

@@ -146,6 +146,7 @@
 /_go-licenses
 /google-chrome
 /google-chrome-stable
+/_gopass
 /_gopherjs
 /_goreleaser
 /gpc

completions/Makefile.am

@@ -706,6 +706,7 @@ CLEANFILES = \
 	_go-licenses \
 	google-chrome \
 	google-chrome-stable \
+	_gopass \
 	_gopherjs \
 	_goreleaser \
 	gpc \
@@ -1120,6 +1121,7 @@ symlinks: $(DATA)
 		_gitsign \
 		_glow \
 		_go-licenses \
+		_gopass \
 		_gopherjs \
 		_goreleaser \
 		_gup \