jshint / jshint / 1570

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Pull #2172

travis-ci

[[FEAT]] Implement new option `futurehostile`

A recent change to JSHint included the ECMAScript 6 global identifiers
in JSHint's collection of "predefined" variables. This disallowed the
definition of those global values even in contexts that did not
implement them.

The intent of this change was to help users avoid bugs when migrating to
the new version of the language. Re-using an existing warning mechanism
caused confusion among many users [1][2][3] for two reasons:

1. The generated warning messages were somewhat misleading (i.e. the
   warning "Redefinition of 'Promise'." is inaccurate in ES5
   environments)
2. The method of disabling the warnings did not accurately communicate
   the intent of the developer (i.e. setting `predef: -Promise` does not
   prompt the user to consider the user to consider if creating their
   own global variable named `Promise` is appropriate)

A dedicated warning message more clearly describes the best practice
that motivates this particular constraint on global variable names. An
explicit option makes it less likely that users will disable this
warning without understanding the motivation.

Intoduce a new option, `futurehostile`, and dedicated warning message to
better communicate the intent of restricting usage of future-reserved
identifiers.

[1] GH-1747 redefinition of Promise (W079)
[2] GH-1995 ES6 globals are activated even though esnext=false
[3] GH-2171 Remove Set from the ECMAScript5 reserved words

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