Allow for option to pass "initialFocusedDate" to DatePicker

[mcMickJuice]

Environment

Tech	Version
material-ui-pickers	1.0.0-rc.9
material-ui	1.2.0
React	16.3.2
Browser	Chrome - 66.0.3359.181
Platform	OSX

Steps to reproduce

1. Set value as null on DatePicker
2. Set minDate to a value greater than today
3. When you open the calendar, the focused date is today

Expected behavior

I would like the ability to provide an "initial focused date" that would be highlighted when I open the picker and I don't have a selected value. If a value is provided, this prop is ignored.

Actual behavior

No ability to specify an initial focused date

Live example

https://codesandbox.io/s/q7jxvqqqq9 - this code example uses an older version, which is where we first noticed the need for this prop (before we upgraded to rc9). This feature is somewhat blocked by #445

[dmtrKovalenko]

@mcMickJuice would you like to work on this and (or) #445 issues and open a PR?

[mcMickJuice]

most definitely 😄

[millnitzluan]

Can I try to do this? Yesterday I did a similar functionality using the calendar.

[dmtrKovalenko]

@millnitzluan of course feel free to open a PR