Support auto detect of file encoding

[mohusheng]

• VSCode Version: 1.7.2
• OS Version: 6.1.7601

[mohusheng]

My project mix gbk and utf8 encoding files, so it need support ["files.encoding": "auto"], no just ["files.encoding": "utf8"] or ["files.encoding": "gbk"].

[kieferrm]

One alternative would be that we remember the encoding a file has been opened before. I.e. when the user chooses Change File Encoding > Reopen with Encoding from the command palette we use the same encoding when the file is opened again.

[visitantz]

Support this request, it's indeed annoying, usually web related files are utf-8 encoding, windows platform files are local encoding, so "files.encoding" setting is not helping at all, you still need to manually set half files each time.

[bpasero]

#5388