merge_asof on multiple columns?
Thank you very much for your work Pandas people! I have the error `pandas.errors.MergeError: can only asof on a key for left` when I run the code below:

```python
df1.sort_values(['gpsLongitude','gpsLatitude'], inplace=True)
df2.sort_values(['x','y'], inplace=True)

res = pd.merge_asof(df1, df2, left_by=['gpsLongitude','gpsLatitude'], right_by=['x','y'] )

```


#### Problem description
it seems that `merge_asof` does not accept list of columns to merge on. [Documentation](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.merge_asof.html) seems to say that the `left_on` argument is a "label" and not a list. 

So maybe what I am asking is for an enhancement? It just seems weird that it can only merge on 1 column. 

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.4.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 36.6.0
Cython: None
numpy: 1.13.3
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.3.1
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.0
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: 2.7.3.1 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
