Attempt to determine the appropriate mode based on the presence of
an interpreter directive on the first line of the file. Interpreter
directives are examined first, followed by the filename when
determining the mode mirroring what Emacs does by default.

1) Constrain the number of characters examine when determining the
   interpreter directive to 256 to avoid performance penalty of scanning
   long lines unnecessarily.
2) Move to a single mode_by_file() function in Context.
3) Always attempt to match the mode based on the filename first,
   falling back to the shebang line only if no match is found.