complete manpages (#534)

* add manpage completions

* fix replacing .gz in man cmp, format braces

custom-completions/man/man-completions.nu

@@ -0,0 +1,16 @@
+def "manpages" [] {
+
+    ^man -w
+	| str trim
+    | split row (char esep)
+	| par-each { glob $'($in)/man?' }
+	| flatten
+	| par-each { ls $in | get name }
+    | flatten
+	| path basename
+	| str replace -s ".gz" ""
+}
+
+export extern "man" [
+    ...targets: string@"manpages"
+]