Modify get_group() method to allow getting multiple groups
#### Problem description

The `get_group()` method supports getting **one group** from a grouped object by
```python
# Current syntax
grouped.get_group('name1')
```
but you can't get **multiple groups** simply by

```python
# Desired syntax
grouped.get_group(['name1', 'name2'])
```
This causes *"ValueError: must supply a tuple to get_group with multiple grouping keys"*

My workaround for now is using `concat` and list comprehension
```python
# Workaround
pd.concat([group for (name, group) in grouped if name in ['name1', 'name2']])
```
but this is a bit cumbersome and not Pythonic...

#### Expected Output

Could we modify `get_group()` to support the syntax shown in code snippet `# Desired syntax`? Or maybe implement another `get_groups()` method?

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.2.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.24.2
pytest: 4.3.1
pip: 19.0.3
setuptools: 40.8.0
Cython: 0.29.6
numpy: 1.16.2
scipy: 1.2.1
pyarrow: None
xarray: 0.12.3
IPython: 7.4.0
sphinx: 1.8.5
patsy: 0.5.1
dateutil: 2.8.0
pytz: 2018.9
blosc: None
bottleneck: 1.2.1
tables: 3.5.1
numexpr: 2.6.9
feather: None
matplotlib: 3.0.3
openpyxl: 2.6.1
xlrd: 1.2.0
xlwt: 1.3.0
xlsxwriter: 1.1.5
lxml.etree: 4.3.2
bs4: 4.7.1
html5lib: 1.0.1
sqlalchemy: 1.3.1
pymysql: None
psycopg2: 2.7.6.1 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
