Add option of opening a fixed and embedded daterange-picker

[MichaelKHTai]

First of all, I must admit it may violate the purpose of daterangepicker. This is because the daterangepicker will no longer be a dropdown, it won't close.

However the following small changes help me a lot while I was required to make a embedded daterangepicker, so I decide to share if someone needs.

Just test it by simply add fixedPicker: true in option, and provide a parentEl which allocate the fixed daterangepicker. You may need to change some style of parentEl, so as to make pretty print.

Bests.