MOFM2T11 — Add an attribute to the files block for users to specify generated file's encoding

MOFM2T aims at being a standard for code generation, and as such it will be used to generate/override files that are under a version control management system. Files can then be generated indiscriminately under windows, linux or mac ... or the three at once for 'big' development teams.

The generated file's encoding then becomes an issue : we cannot always generate files with the default system's encoding : windows' CP1252 as no equivalent in unices, and unix' UTF-8 might cause problems in windows if windows developpers re-generated files that have been first generated under an unix system.

File encoding should be an optional attribute of the file blocks; so someone could write
[file ('class.java', false, 'UTF-16')] or [file ('class.java', false, 'ISO-8859-1')] to tell that the target file should be encoded in such or such way.

Note : this goes along with a previously raised issue in which I asked that the file's unique ID be removed. we'll need to find how to describe the encoding if the unique ID is not removed (as that would make file blocks with two optionnal expressions as their last parameter).