[POC] Allow string default position

[eric-burel]

Hi,
Since react-draggable relies on transform, it can sometimes be difficult to create components that rely themselves on transform, since transformations will be overidden by react-draggable.

This PR especially solves #223. Indeed, the only way to center a floating div on the screen is to use a transform(-50%, -50%).

So, I added the possibility to provide the defaultPosition as a string.

Cool stuffs:

• we can provide a relative defaultPosition, this is good for centering
• it does the job

Bad stuffs:

• it breaks automatic tests and sorry, I lack the time to update them (the live example is working though)
• it relies on calc, I am not sure if this is good / cross-browser / IE compatible
• it add computations in the transform generation, which reduce performance (but is it significant ?), instead of using the defaultPosition only during the component construction.
• it is not optimized, I put calc everytime even if all values are string

[STRML]

Hi - apologies that master is now conflicting, could you make the following changes:

1. Don't commit dist/, and
2. Port your changes from the old .es6 files (this extension had issues with some tools like flow-upgrade) over to .js.

Thanks for your contribution! 👍

[tnrich]

@eric-burel can you please finish this PR? I would also like to see this work!

@STRML please please make this PR work

[tnrich]

I made #361 which fixes @STRML's above comments.

[eric-burel]

Thanks @tnrich, I close this PR since you opened a new one.