Enable custom SwiftPlate repo to be specified as command line argument

With this change, a custom SwiftPlate repo (say, a local clone), can be used
to get the template when running SwiftPlate. This is useful both for development
of SwiftPlate, but also in case someone wants to supply their own template.

Usage:

```
$ swiftplate --repo /users/john/code/swiftplate 
```

main.swift

@@ -76,6 +76,7 @@ struct Arguments {
     var authorEmail: String?
     var githubURL: String?
     var organizationName: String?
+    var repositoryURL: URL?
 
     init(commandLineArguments arguments: [String]) {
         for (index, argument) in arguments.enumerated() {
@@ -92,6 +93,10 @@ struct Arguments {
                 githubURL = arguments.element(after: index)
             case "--organizationname":
                 organizationName = arguments.element(after: index)
+            case "--repo":
+                if let urlString = arguments.element(after: index) {
+                    repositoryURL = URL(string: urlString)
+                }
             default:
                 break
             }
@@ -212,10 +217,10 @@ func askForDestination() -> String {
     return fileManager.currentDirectoryPath
 }
 
-func performGitClone(path: String) throws {
+func performGitClone(url: URL, path: String) throws {
     let process = Process()
     process.launchPath = "/bin/bash"
-    process.arguments = ["-c", "git clone https://github.com/JohnSundell/SwiftPlate.git '\(path)' -q"]
+    process.arguments = ["-c", "git clone \(url.absoluteString) '\(path)' -q"]
     process.launch()
     process.waitUntilExit()
 }
@@ -279,7 +284,8 @@ do {
     }
 
     try performCommand(description: "Making a local clone of the SwiftPlate repo") {
-        try performGitClone(path: gitClonePath)
+        let repositoryURL = arguments.repositoryURL ?? URL(string: "https://github.com/JohnSundell/SwiftPlate.git")!
+        try performGitClone(url: repositoryURL, path: gitClonePath)
     }
 
     try performCommand(description: "Copying template folder") {