BUG: concat of ExtensionArray with other dtype fails
```
In [22]: from pandas.tests.extension.decimal.array import DecimalArray, make_data
    ...: dec_arr = DecimalArray(make_data())
    ...: df1 = pd.DataFrame({'ext': dec_arr[:3]})

In [23]: df2 = pd.DataFrame({'ext': [1.0, 2.0]})

In [24]: pd.concat([df1, df2], axis=0)
---------------------------------------------------------------------------
AxisError                                 Traceback (most recent call last)
<ipython-input-24-e046f4a55b52> in <module>()
----> 1 pd.concat([df1, df2], axis=0)

/home/joris/scipy/pandas/pandas/core/reshape/concat.py in concat(objs, axis, join, join_axes, ignore_index, keys, levels, names, verify_integrity, copy)
    211                        verify_integrity=verify_integrity,
    212                        copy=copy)
--> 213     return op.get_result()
    214 
    215 

/home/joris/scipy/pandas/pandas/core/reshape/concat.py in get_result(self)
    406             new_data = concatenate_block_managers(
    407                 mgrs_indexers, self.new_axes, concat_axis=self.axis,
--> 408                 copy=self.copy)
    409             if not self.copy:
    410                 new_data._consolidate_inplace()

/home/joris/scipy/pandas/pandas/core/internals.py in concatenate_block_managers(mgrs_indexers, axes, concat_axis, copy)
   5415             print("--- Using concatenate_join_units")
   5416             b = make_block(
-> 5417                 concatenate_join_units(join_units, concat_axis, copy=copy),
   5418                 placement=placement)
   5419         blocks.append(b)

/home/joris/scipy/pandas/pandas/core/internals.py in concatenate_join_units(join_units, concat_axis, copy)
   5557             concat_values = concat_values.copy()
   5558     else:
-> 5559         concat_values = _concat._concat_compat(to_concat, axis=concat_axis)
   5560 
   5561     return concat_values

/home/joris/scipy/pandas/pandas/core/dtypes/concat.py in _concat_compat(to_concat, axis)
    189                 to_concat = [x.astype('object') for x in to_concat]
    190 
--> 191     return np.concatenate(to_concat, axis=axis)
    192 
    193 

AxisError: axis 1 is out of bounds for array of dimension 1
```

This is because `_concat_compat` is not yet aware of extension arrays, and hence does not reshape the extension array object values to 2D (as is done correctly in case of other non-consolidatable blocks like datetimetz)