Skip alias versions, add --with-aliases option.

By default, skip ruby versions that are aliases. I'm having a hard time
thinking of any reason you would want to run the command on the same
ruby version multiple times by default.

Also tweak the option parsing and usage messages in support of this
change.

With versions `2.1 => 2.1.7`, `2.1.6`, and `2.1.7`:

    $ rbenv each ruby --version
    ruby 2.1.6p336 (2015-04-13 revision 50298) [x86_64-linux]
    ruby 2.1.7p400 (2015-08-18 revision 51632) [x86_64-linux]

    $ rbenv each --with-aliases ruby --version
    ruby 2.1.7p400 (2015-08-18 revision 51632) [x86_64-linux]
    ruby 2.1.6p336 (2015-04-13 revision 50298) [x86_64-linux]
    ruby 2.1.7p400 (2015-08-18 revision 51632) [x86_64-linux]

README.md

@@ -22,4 +22,5 @@ the output.
 ```
 $ rbenv each bundle install
 $ rbenv each -v rake test
+$ rbenv each --with-aliases bundle check
 ```

bin/rbenv-each

@@ -2,36 +2,57 @@
 #
 # Summary: Execute a command for each Ruby version
 #
-# Usage: rbenv each [-v] <command> [arg1 arg2...]
+# Usage: rbenv each [options] <command> [arg1 arg2...]
 #
-# Executes a command for each Ruby version by setting RBENV_VERSION.
+# Execute a command for each Ruby version by setting RBENV_VERSION.
 # Failures are collected and reported at the end.
 #
-#    -v  Verbose mode. Prints a header for each ruby.
+#
+# Options:
+#    -h, --help           Print this help message
+#    -a, --with-aliases   Don't skip ruby versions that are aliases (symlinks)
+#    -v, --verbose        Verbose mode: print a header for each ruby version
 #
 set -e
 [ -n "$RBENV_DEBUG" ] && set -x
 
-# Provide rbenv completions
-case "$1" in
-  --complete )
-    echo --help
-    echo --verbose
-    exit
-    ;;
-  -v | --verbose )
-    verbose=1
-    shift
-    ;;
-  --help )
-    rbenv-help each
-    exit
-    ;;
-  ""  | -* )
-    rbenv-help --usage each >&2
-    exit 1
-    ;;
-esac
+while [[ $1 == -* ]]; do
+  case "$1" in
+    --)
+      shift
+      break
+      ;;
+
+    # Provide rbenv completions
+    --complete )
+      echo --help
+      echo --verbose
+      echo --with-aliases
+      exit
+      ;;
+    -a | --with-aliases )
+      aliases=1
+      ;;
+    -v | --verbose )
+      verbose=1
+      shift
+      ;;
+    -h | --help )
+      rbenv-help each
+      exit
+      ;;
+    ""  | -* )
+      rbenv-help --usage each >&2
+      exit 1
+      ;;
+  esac
+  shift
+done
+
+if [ $# -lt 1 ]; then
+  rbenv-help --usage each >&2
+  exit 1
+fi
 
 GRAY=""
 RED=""
@@ -49,7 +70,13 @@ failed_rubies=""
 
 trap "exit 1" INT
 
-for ruby in $(rbenv versions --bare); do
+if [ -n "$aliases" ]; then
+  opts=
+else
+  opts="--skip-aliases"
+fi
+
+for ruby in $(rbenv versions --bare $opts); do
   if [ -n "$verbose" ]; then
     header="===[$ruby]==================================================================="
     header="${header:0:72}"