Hints: handle up and down targets

It's apparently easy to accidentally add hints. One glyph contained in its lib field:

```
{'com.schriftgestaltung.Glyphs.glyph.rightMetricsKey': '=|',
'com.schriftgestaltung.hints': [{'horizontal': True, 'options': 0, 'origin': [0, 2], 'stem': -2, 'target': ['d', 'o', 'w', 'n'], 'type': 'TopGhost'}],
'public.markColor': '0.04,0.57,0.04,1'}
```

glyphsLib output:
```
Traceback (most recent call last):
...
File "/Users/.../venv/lib/python3.6/site-packages/glyphsLib/builder/__init__.py", line 115, in to_glyphs
return builder.font
File "/Users/.../venv/lib/python3.6/site-packages/glyphsLib/builder/builders.py", line 343, in font
self.to_glyphs_glyph(glyph, layer, master)
File "/Users/.../venv/lib/python3.6/site-packages/glyphsLib/builder/glyph.py", line 216, in to_glyphs_glyph
self.to_glyphs_hints(ufo_glyph, layer)
File "/Users/.../venv/lib/python3.6/site-packages/glyphsLib/builder/hints.py", line 57, in to_glyphs_hints
value = Point(*hint[attr])
TypeError: __init__() takes from 1 to 4 positional arguments but 5 were given
```

https://github.com/googlei18n/glyphsLib/blob/b35f4abe16df4d49f687833dbf841b9d7b0ec281/Lib/glyphsLib/builder/hints.py#L55

Start at Lib/glyphsLib/builder/hints.py around line 55 and reproduce the conversion using a glyph containing a hint whose target is ['d', 'o', 'w', 'n']. Confirm the current Point construction failure, then verify that handling these targets completes without the TypeError and preserves the hint data.

Written by the indexing model from the issue text.