Add a requirement on iconv and suggest mbstring in composer.json

The iconv PHP extension is used in `ParserState` and hence needs to be
required.

The mbstring extension is optional, but helpful. So we should help
users discover this with a `suggest` entry in the `composer.json`.

Author: oliverklee

composer.json

@@ -15,12 +15,16 @@
         }
     ],
     "require": {
-        "php": ">=5.6.20"
+        "php": ">=5.6.20",
+        "ext-iconv": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "^4.8.36",
         "codacy/coverage": "^1.4"
     },
+    "suggest": {
+        "ext-mbstring": "for parsing UTF-8 CSS"
+    },
     "autoload": {
         "psr-4": {
             "Sabberworm\\CSS\\": "src/"