Add properties output as a --format

When having big outputs as json or array structures it is sometimes hard to preserve context.

By allowing output format like

drush etr file --format=properties
file.label = File
file.base table = file_managed
file.entity keys.id = fid
file.entity keys.label = filename
file.entity keys.revision = 
file.entity keys.bundle = 
file.static cache = 
file.fieldable = 
file.controller class = DrupalDefaultEntityController
file.field cache = 1
file.load hook = file_load
file.bundles.file.label = File
file.schema_fields_sql.base table.0 = fid
...
file.schema_fields_sql.base table.7 = timestamp

it is easier to follow and to grep like

drush er node 1 2 3 4 5 6 --format=properties | grep format
1.body.und.0.format = plain_text
2.body.und.0.format = full_html
3.body.und.0.format = filtered_html
4.body.und.0.format = full_html
5.body.und.0.format = filtered_html
6.body.und.0.format = plain_text

Related to #1366098: Automatically download Symfony YAML component but a mere necessity when trying to grasp output from #1005480: Create a new project to develop Drush entity support

The patch follows the key = value format taken from ie http://en.wikipedia.org/wiki/.properties