Feat/resize columns

[mir4a]

This PR adds possibility to resize columns with with two callbacks: for tracking resize (dragging) and resize end useful for storing which columns were resized. Stored values then can be passed as a property so that after reloading the page the table will restore touched column widths.

What is done:

• [x] columns are resizable
• [x] callback for dragging
• [x] callback for resize end
• [x] accept initial values for columns
• [x] re-calculate table's height when horizontal scroll appears
• [x] prevent column collapsing when resized. Partially done by setting min (resizable) width to 40px
• [ ] adjust the rest column widths to fit available space so that the right border of the table doesn't look broken

If you would like to test it out right away, you can try my fork published on npm: npm i @mir4a/react-datasheet-grid@4.12.0-alpha.15 --save-exact.

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[tdonkena]

@nick-keller Can this get reviewed and merged please, this has been open for 2 months