fix(util.breakText): use the specified separator to join words, allow…

[kumilingus]

… empty string to be separator; fix(dia.attributes): textWrap reads all breakText options

Description

When using a custom separator, the space character was always used to join words. This PR uses a separator on the input to join words as long as the separator was a string.

The separator can now be an empty string.

const text = joint.util.breakText('first*second', { width: 1000 }, {}, { separator: '*' });
// Assert: text === 'first*second'
// Prior to this PR the `first second` string has been returned

The PR also makes sure that textWrap cell attribute accepts the separator option.

element.attr('label/textWrap/separator', '*');