[DX] Improve dumpAssets for composer update & install

Possible approach for fixing dx issue with asset dumping and composer update.

Fixes:
- crashes when interfaces or something else needs to be cleared from prod cache
- control over when assets are dumped, as it is not needed on updates of most composer packages

Patch for ezpublish-community:
```diff
diff --git a/composer.json b/composer.json
index 3c4fd1e..b01ba6c 100644
--- a/composer.json
+++ b/composer.json
@@ -91,9 +91,9 @@
             "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::clearCache",
             "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installAssets",
             "eZ\\Bundle\\EzPublishLegacyBundle\\Composer\\ScriptHandler::installAssets",
-            "eZ\\Bundle\\EzPublishCoreBundle\\Composer\\ScriptHandler::dumpAssets",
             "Sensio\\Bundle\\DistributionBundle\\Composer\\ScriptHandler::installRequirementsFile",
-            "eZ\\Bundle\\EzPublishLegacyBundle\\Composer\\ScriptHandler::installLegacyBundlesExtensions"
+            "eZ\\Bundle\\EzPublishLegacyBundle\\Composer\\ScriptHandler::installLegacyBundlesExtensions",
+            "eZ\\Bundle\\EzPublishCoreBundle\\Composer\\ScriptHandler::dumpAssetsHelpText"
         ]
     },
     "config": {
```


Note: this does not mean doing composer update in production is safe, such changes should still be staged as you'll often need to also clear prod cache as well and other changes like db updates.


## Screen shoots
_NB: Text slightly updated after screen shoots where taken._

##### Help text for composer update:
![screen shot 2014-10-20 at 17 16 52](https://cloud.githubusercontent.com/assets/289757/4703686/cf2a73f6-586c-11e4-9e52-85edf4fe137c.png)

##### Prompt for composer install:
![screen shot 2014-10-20 at 17 16 29](https://cloud.githubusercontent.com/assets/289757/4703694/ddbf626e-586c-11e4-9417-c6b80f51759a.png)
Doesn't it work with command XML tags ? e.g. `<comment>blabla</comment>`?