Add initWidth argument to colDef

[nteetor]

Description

This PR adds an initWidth column to the colDef function. As described in #223 and #374 there isn't currently a way to specify a column's initial width and allow column resizing.

I adjusted the javascript column logic to set width to initWidth, if specified. The behavior of the other width arguments remain unchanged.

Changes

• Added initWidth argument to colDef()
• Added argument checks in colDef() for the following,
  • initWidth is null or numeric
  • initWidth and width cannot both be specified
  • if specified, initWidth is greater than or equal to minWidth
  • if specified, initWidth is less than or equal to minWidth
• Added R tests
  • confirm argument checks listed above
• Added Javascript tests
  • confirm initial width does not affect min or max width
  • initial width leaves a resizable column as resizable