text: Title: mode on `axis=1`
  Body: The `mode` method in a `dask` `DataFrame` does not allow for the argument `axis=1`. It would be great to have since it seems that in `pandas`, that operation is very slow and seems straightforward to parallelize. 
  I would like to be able to do this in dask.
  ```
  import pandas as pd
  import numpy as np
  import dask.dataframe as dd
  np.random.seed(0)
  N_ROWS = 1_000
  df = pd.DataFrame({'a':np.random.randint(0, 100, N_ROWS),
                'b':np.random.randint(0, 100, N_ROWS),
                'c':np.random.randint(0, 100, N_ROWS)})
  df['d'] = df['a'] #ensure mode is column 'a', unless b=c, then there are two modes
  
  df.mode(axis=1)
  ```
  
  For reference, in pandas with `N_ROWS = 100_000`, the mode operation takes 20 seconds, and the time seems to grow linearly with number of observations. 