Allow writers to create output directories if they don't exist

[djhoese]

If a user specifies a directory in their filename pattern it is not currently created which will causes errors in most readers. Example:

    scn.save_datasets(filename='{start_time:%Y%m%d}/{name}_{start_time:%Y%m%d_%H%M%S}.tif')

Which should create a 20180101 directory and put a C01_20180101_120000.tif file in it.

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[coveralls]

Coverage increased (+0.06%) to 74.16% when pulling f90ac1e on djhoese:bugfix-output-dir-creation into e2a3ac7 on pytroll:master.

[coveralls]

Coverage increased (+0.05%) to 74.156% when pulling 84195be on djhoese:bugfix-output-dir-creation into 82d7cbc on pytroll:master.

[codecov]

Codecov Report

Merging #478 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #478   +/-   ##
======================================
  Coverage    74.1%   74.1%           
======================================
  Files         136     136           
  Lines       18036   18036           
======================================
  Hits        13366   13366           
  Misses       4670    4670

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[djhoese]

@mraspaud I asked for you to review this PR but wanted to merge it as soon as possible because I wanted to see if my small change to setup.py fixes the readthedocs build. Feel free to still comment on anything you disagree with and I'll fix it tomorrow.

[djhoese]

It did fix readthedocs!