config: add defaultvalue template keyword
This patch tries to fix one of the issues mentioned in issue6014.
This adds a new `defaultvalue` template keyword to be used with
`hg showconfig` to get the default value of the config item.
Differential Revision: https://phab.mercurial-scm.org/D6704
{
// Enforcing
"eqeqeq" : true, // true: Require triple equals (===) for comparison
"forin" : true, // true: Require filtering for..in loops with obj.hasOwnProperty()
"freeze" : true, // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
"nonbsp" : true, // true: Prohibit "non-breaking whitespace" characters.
"undef" : true, // true: Require all non-global variables to be declared (prevents global leaks)
// Environments
"browser" : true // Web Browser (window, document, etc)
}