Add support of the Cache.SerializerPermissions for default profile

[OxCom]

Allow to setup default file permissions for HTMLPurifier. See configuration in docs.

Note: SerializerPermissions must be set as integer that will be converted to correct chmod permissions. Example: 493 int => 0755 oct and 511 int => 0777 oct

[validaide-mark-bijl]

This seems like a sensible extension! And BTW, you mentioning that permissions need to be defined as a decimal int to be interpreted as an octal just saved my day!

[HeahDude]

This is a very nice contribution, thank you @OxCom!

[HeahDude]

I've just tagged 4.1.