support magic comments for specifying file encoding

[vladdu]

Some programming languages use magic comments at the beginning of a file to specify the encoding of that file. From what I can see, this is not supported by vscode. I would like to suggest to have it implemented. It is a problem when one has projects where there are multiple encodings used across the code base.

[vladdu]

Eclipse has a nice mechanism where plugins can provide an encoding detector in code, but maybe it is enough to support searching for a magic regexp in the first two lines, as specified for example in https://www.python.org/dev/peps/pep-0263/

[alexdima]

fyi Benjamin Pasero (@bpasero)