Implement a break-trigger for incomplete expressions in IEx. Closes #1089
Call it simply break-trigger in the docstring
Document the break-trigger in `h/0`
Change the break-trigger for IEx to #iex:break
Merge pull request #1096 from alco/iex-break-1089

Implement a break-trigger for incomplete expressions in IEx.
Improve docs for the IEx module and some error messages as well