Custom escape function, single-quotes in nested

[aemino]

• Adds the option to provide a custom escape function for interpolation
• Converts single-quotes in nesting to double-quotes for JSON parsing

[coveralls]

Coverage decreased (-0.5%) to 65.017% when pulling a45efb4 on Programmix:patch-1 into 90254b0 on i18next:master.

[jamuhl]

Would you mind adding a test: https://github.com/i18next/i18next/blob/master/test/interpolation.spec.js#L156

[coveralls]

Coverage increased (+0.9%) to 66.398% when pulling a3c899b on Programmix:patch-1 into 90254b0 on i18next:master.

[jamuhl]

thank you...merged and published in i18next@4.1.0