Fix #1273, Handle backslashes at the end of heredocs

Fixes #1273. There are two main fixes to master:
```coffee
[
  '\
  '     # didn't compile at all
  "\
  "
  '''\
  '''
  """\
  """
] # the rest did compile to ["\  ", '\', "\"];
```
now
```javascript
['', "", '', ""];
```
Second, escaping quotes at the end of heredoc:
```coffee
"""\"""
```
used to compile to `"\";`. Now throws as expected:
```
error: missing ", starting
"""\"""
      ^
```
And I also fixed trailing backslashes (hence the added tests).