Allow to export commands with list in files

[nenes25]

As symfony default command list option we should export the result of the list in the following formats :

1. Txt
2. Xml
3. Json
4. Md

The affected commands are the following :

• admin:user:list
• dev:cron:list
• dev:list-overrides
• hook:list
• module:hook:list
• module:list
• module:tab:list

[m3uusi]

Some POC done in the commit:
m3uusi@598d16b

Maybe a Trait could be more interesting than an abstract class.

It could be interesting too to take a decision concerning the display of all list and harmonize all list command :

• When displaying the results in text mode sometimes we have an header, sometimes not.
• When there is not data (in json/xml format) , should we display an info or format an empty data object in the right format ?

The development I made was not based on the 1.7 branch so the namespace are still incorrect.
Waiting for answers to port it on the right branch with the right modifications.