add --pure-funcs option

it has the same effect as specifying `pure_funcs` in `--compressor`
option, however it's much easier to use

instead of:

    --compressor 'pure_func=["Math.floor","debug","console.logTime"]'

it's now possible:

    --compressor --pure-funcs Math.floor debug console.logTime

fixes #684

README.md

@@ -134,6 +134,8 @@ The available options are:
   --mangle-props                Mangle property names
   --mangle-regex                Only mangle property names matching the regex
   --name-cache                  File to hold mangled names mappings
+  --pure-funcs                  List of functions that can be safely removed if
+                                their return value is not used           [array]
 ```
 
 Specify `--output` (`-o`) to declare the output file.  Otherwise the output