turns 'undefined' objects in strings

[dominictarr]

var f = new (require('fomatto').Formatter)()
f('qq_{x}_qq', {x: undefined})

returns 'qq_[object Object]_qq'

which is not right, should work the same as:

var f = new (require('fomatto').Formatter)()
f('qq_{0}_qq', undefined)

which returns 'qq_undefined_qq'

[BonsaiDen]

Fixed a0ab78c