add fill_func to fillna #1

[ghk829]

What does this PR do?

I add the parameters a fill_func for filling NA

default is the same as the original
and
we can expand the function to fill the NAs

Where should the reviewer start?

the constructer of the autoclean function

How should this PR be tested?

I add the test function in tests.py

Any background context you want to provide?

python dynamically import and numpy nanfunctions modules

What are the relevant issues?

#1

Screenshots (if appropriate)

Questions:

• Do the docs need to be updated?
  If my pull request is admitted, I will add docs
• Does this PR add new (Python) dependencies?
  Nope

[ghk829]

Travis Ci build failed for the reasons.
Can I fix the issues?

[coveralls]

Coverage increased (+1.1%) to 80.377% when pulling 69aeb92 on ghk829:development into f6f92d7 on rhiever:development.