accept any file suffix as YAML file

[vetinari]

check if it's an existing file and try to load it

fixes #58

[codecov[bot]]

Codecov Report

Merging #60 into master will decrease coverage by 0.63%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
- Coverage   74.13%   73.49%   -0.64%     
==========================================
  Files          18       18              
  Lines        1040     1049       +9     
==========================================
  Hits          771      771              
- Misses        269      278       +9

Impacted Files	Coverage Δ
zmon_cli/cmds/entity.py	40.5% <0%> (-5.21%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update e60389e...1ff1b2a. Read the comment docs.

[vetinari]

:+1: