add well-known bucket functionality (only extras so far)

buckets.json

@@ -0,0 +1,3 @@
+{
+	"extras": "https://github.com/lukesampson/scoop-extras.git"
+}

lib/buckets.ps1

@@ -4,6 +4,13 @@ function bucketdir($name) {
     "$bucketsdir\$name"
 }
 
+function known_bucket_repo($name) {
+    $dir = versiondir 'scoop' 'current'
+    $json = "$dir\buckets.json"
+    $buckets = gc $json -raw | convertfrom-json -ea stop
+    $buckets.$name
+}
+
 <#
 # convert an object to a hashtable
 function hashtable($obj) {

libexec/scoop-bucket.ps1

@@ -25,7 +25,10 @@ $usage_rm = "usage: scoop bucket rm <name>"
 
 function add_bucket($name, $repo) {
     if(!$name) { "<name> missing"; $usage_add; exit 1 }
-    if(!$repo) { "<repo> missing"; $usage_add; exit 1 }
+    if(!$repo) {
+        $repo = known_bucket_repo $name
+        if(!$repo) { "unknown bucket '$name': try specifying <repo>"; $usage_add; exit 1 }
+    }
 
     $git = try { gcm 'git' -ea stop } catch { $null }
     if(!$git) {