Feature request: DataFrame.mode(dropna=False)

[fulldecent]

Code Sample, a copy-pastable example if possible

Current:

df = pd.DataFrame({'A': [1, 2, np.nan, np.nan, np.nan]})
df.mode()

     A
0  1.0
1  2.0

Proposed:

df = pd.DataFrame({'A': [1, 2, np.nan, np.nan, np.nan]})
df.mode(dropna=False)

     A
0   NA

Problem description

Current behavior is inconsistent with value_counts() because that allows value_counts(dropna=False).

Output of pd.show_versions()

[paste the output of pd.show_versions() here below this line]
INSTALLED VERSIONS

commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.3
pytest: None
pip: 9.0.1
setuptools: 32.2.0
Cython: None
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
pandas_gbq: None
pandas_datareader: None

[TomAugspurger]

Seems reasonable. Are you interested in submitting a PR for it?

[fulldecent]

Well here's what I have so far #17547