fix(ngList): use custom separators for re-joining list items

The separator string used to split the view value into a list for the model
value is now used to join the list items back together again for the view value.

BREAKING CHANGE:

The `ngList` directive no longer supports splitting the view value
via a regular expression. We need to be able to re-join list items back
together and doing this when you can split with regular expressions can
lead to inconsistent behaviour and would be much more complex to support.

If your application relies upon ngList splitting with a regular expression
then you should either try to convert the separator to a simple string or
you can implement your own version of this directive for you application.

Closes #4008
Closes #2561
Closes #4344
feat(ngList): use ngTrim to manage whitespace handling when splitting

With the removal of regular expression support `ngList` no longer supported
splitting on newlines (and other pure whitespace splitters).

This change allows the application developer to specify whether whitespace
should be respected or trimmed by using the `ngTrim` attribute. This also
makes `ngList` consistent with the standard use of `ngTrim` in input directives
in general.

Related To: #4344
test(ngList): add protractor test to example