Add installer script

It works via `script/install.sh PLUGIN_TYPE` command.
Supported plugin types are  `Debug`, `Beta` and `Test`.
Type argument can be omitted to replace original plugin.

script/install.sh

@@ -0,0 +1,49 @@
+#!/bin/bash
+set -e
+
+main ()
+{
+    SKETCH_APP_PATH="/Applications/Sketch.app"
+    SKETCH_PLUGIN_DIR="$HOME/Library/Application Support/com.bohemiancoding.sketch3/Plugins"
+    SCRIPT_DIR=$(cd "$(dirname "$0")"; pwd)
+    PLUGIN_PATH="$SCRIPT_DIR/../Zeplin.sketchplugin"
+
+    TYPES=('Debug' 'Beta' 'Test')
+
+    if [[ " ${TYPES[*]} " == *"$1"* ]]; then
+        if [ -z "$1" ]; then
+            IDENTIFIER_SUFFIX=""
+            NAME_SUFFIX=""
+        else
+            IDENTIFIER_SUFFIX=".$(echo "$1" | tr '[:upper:]' '[:lower:]')"
+            NAME_SUFFIX=" $(tr '[:lower:]' '[:upper:]' <<< ${1:0:1})$(tr '[:upper:]' '[:lower:]' <<< ${1:1})"
+        fi  
+    else
+        echo "Argument must be one of the followings: ${TYPES[@]}"
+
+        exit 0
+    fi
+
+    TEMP_PLUGIN_PATH="$TMPDIR/Zeplin$NAME_SUFFIX.sketchplugin"
+    SKETCH_PLUGIN_PATH="$SKETCH_PLUGIN_DIR/Zeplin$NAME_SUFFIX.sketchplugin"
+
+    rm -rf "$TEMP_PLUGIN_PATH"
+    cp -r "$PLUGIN_PATH" "$TEMP_PLUGIN_PATH"    
+
+    sed -i '' -e '/"name": "/s/Zeplin/Zeplin'"$NAME_SUFFIX"'/' "$TEMP_PLUGIN_PATH/Contents/Sketch/manifest.json"
+    sed -i '' -e '/"identifier": "/s/io.zeplin.sketch-plugin/io.zeplin.sketch-plugin'"$IDENTIFIER_SUFFIX"'/' "$TEMP_PLUGIN_PATH/Contents/Sketch/manifest.json"
+    sed -i '' -e 's/io.zeplin.osx/io.zeplin.osx'"$IDENTIFIER_SUFFIX"'/' "$TEMP_PLUGIN_PATH/Contents/Sketch/helpers.cocoascript"
+
+    if [ -d "$SKETCH_PLUGIN_PATH" ]; then
+        rm -rf "$SKETCH_PLUGIN_PATH"
+        cp -r "$TEMP_PLUGIN_PATH" "$SKETCH_PLUGIN_PATH"
+
+        open "$SKETCH_APP_PATH"
+    else
+        open "$TEMP_PLUGIN_PATH"
+    fi
+
+    exit 0
+}
+
+main "$1"