Refine "ABS: Compile File" command

[rudi]

• Handle Emacs-style in-file option to override the default backend
• Handle Emacs-style in-file option to specify other files to be included in compilation

These options can take one of two forms: either a commend containing a group of options on the first line of the file:

// -*- abs-target-language: java; abs-input-files: ("file1.abs" "file2.abs"); -*-

Or a comment block at the end of the file:

// Local Variables:
// abs-target-language: java
// abs-input-files: ("file1.abs" "file2.abs")
// End: