Refactor TakesArgs to use an interface for arg validation.

Fix some typos in README and comments.
Move arg validation to after flag validation so that the help flag is run first.
Pass the same args to ValidateArgs as the Run methods receive.
Update README.

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

README.md

@@ -467,36 +467,34 @@ A flag can also be assigned locally which will only apply to that specific comma
 RootCmd.Flags().StringVarP(&Source, "source", "s", "", "Source directory to read from")
 ```
 
-### Specify if you command takes arguments
+## Positional and Custom Arguments
 
-There are multiple options for how a command can handle unknown arguments which can be set in `TakesArgs`
-- `Legacy`
-- `None`
-- `Arbitrary`
-- `ValidOnly`
+Validation of positional arguments can be specified using the `Args` field.
 
-`Legacy` (or default) the rules are as follows:
-- root commands with no subcommands can take arbitrary arguments
-- root commands with subcommands will do subcommand validity checking
-- subcommands will always accept arbitrary arguments and do no subsubcommand validity checking
+The follow validators are built in:
 
-`None` the command will be rejected if there are any left over arguments after parsing flags.
+- `NoArgs` - the command will report an error if there are any positional args.
+- `ArbitraryArgs` - the command will accept any args.
+- `OnlyValidArgs` - the command will report an error if there are any positional args that are not in the ValidArgs list.
+- `MinimumNArgs(int)` - the command will report an error if there are not at least N positional args.
+- `MaximumNArgs(int)` - the command will report an error if there are more than N positional args.
+- `ExactArgs(int)` - the command will report an error if there are not exactly N positional args.
+- `RangeArgs(min, max)` - the command will report an error if the number of args is not between the minimum and maximum number of expected args.
 
-`Arbitrary` any additional values left after parsing flags will be passed in to your `Run` function.
-
-`ValidOnly` you must define all valid (non-subcommand) arguments to your command. These are defined in a slice name ValidArgs. For example a command which only takes the argument "one" or "two" would be defined as:
+A custom validator can be provided like this:
 
 ```go
-var HugoCmd = &cobra.Command{
-        Use:   "hugo",
-        Short: "Hugo is a very fast static site generator",
-	ValidArgs: []string{"one", "two", "three", "four"}
-	TakesArgs: cobra.ValidOnly
-        Run: func(cmd *cobra.Command, args []string) {
-            // args will only have the values one, two, three, four
-	    // or the cmd.Execute() will fail.
-        },
-    }
+
+Args: func validColorArgs(cmd *cobra.Command, args []string) error {
+  if err := cli.RequiresMinArgs(1)(cmd, args); err != nil {
+    return err
+  }
+  if myapp.IsValidColor(args[0]) {
+     return nil
+  }
+  return fmt.Errorf("Invalid color specified: %s", args[0])
+}
+
 ```
 
 ### Bind Flags with Config
@@ -517,6 +515,7 @@ when the `--author` flag is not provided by user.
 
 More in [viper documentation](https://github.com/spf13/viper#working-with-flags).
 
+
 ## Example
 
 In the example below, we have defined three commands. Two are at the top level