Allow running pastebin with the URL

For instance, `pastebin run https://pastebin.com/LYAxmSby` will now
extract the code and download appropriately. Also add an error message
when we received something which is not a valid pastebin code.

See #134.

src/main/resources/assets/computercraft/lua/rom/programs/http/pastebin.lua

@@ -18,7 +18,32 @@ if not http then
     return
 end
 
-local function get(paste)
+--- Attempts to guess the pastebin ID from the given code or URL
+local function extractId(paste)
+    local patterns = {
+        "^([%a%d]+)$",
+        "^https?://pastebin.com/([%a%d]+)$",
+        "^pastebin.com/([%a%d]+)$",
+        "^https?://pastebin.com/raw/([%a%d]+)$",
+        "^pastebin.com/raw/([%a%d]+)$",
+    }
+
+    for i = 1, #patterns do
+        local code = paste:match( patterns[i] )
+        if code then return code end
+    end
+
+    return nil
+end
+
+local function get(url)
+    local paste = extractId( url )
+    if not paste then
+        io.stderr:write( "Invalid pastebin code.\n" )
+        io.write( "The code is the ID at the end of the pastebin.com URL.\n" )
+        return
+    end
+
     write( "Connecting to pastebin.com... " )
     -- Add a cache buster so that spam protection is re-checked
     local cacheBuster = ("%x"):format(math.random(0, 2^30))

src/test/resources/test-rom/spec/apis/http_spec.lua

@@ -1,17 +1,17 @@
 describe("The http library", function()
     describe("http.checkURL", function()
-        it("Accepts well formed domains", function()
+        it("accepts well formed domains", function()
             expect({ http.checkURL("https://google.com")}):same({ true })
         end)
 
-        it("Rejects malformed URLs", function()
+        it("rejects malformed URLs", function()
             expect({ http.checkURL("google.com")}):same({ false, "Must specify http or https" })
             expect({ http.checkURL("https:google.com")}):same({ false, "URL malformed" })
             expect({ http.checkURL("https:/google.com")}):same({ false, "URL malformed" })
             expect({ http.checkURL("wss://google.com")}):same({ false, "Invalid protocol 'wss'" })
         end)
 
-        it("Rejects local domains", function()
+        it("rejects local domains", function()
             expect({ http.checkURL("http://localhost")}):same({ false, "Domain not permitted" })
             expect({ http.checkURL("http://127.0.0.1")}):same({ false, "Domain not permitted" })
         end)