Align the Vim syntax file to match the Mercury language more closely

vim/README:
    Removed warning about ~/.vim/after/filetype.vim,

    Add utf-8 encoding detection.

    Added more details to the Vim README and

vim/after/filetype.vim:
vim/ftdetect/mercury.vim.
    Renamed file type detection for Mercury to ftdetect/mercury.vim.  This
    allows one to install the vim configuration files with a single command:
    $ cp -r vim/ ~/.vim

vim/syntax/mercury.vim:
    Fixed escaping of the escape character in strings,
vim/README fixes.

+ Expand tabs to spaces.
+ "set modeline" is optional.

vim/README:
    As above.