Implement Predicate decorator

[doloopwhile]

Hello. I added Predicate decorator.

Currently, To define a functional schema, We have to write a def statement and an assignment statement.

def some_condition(x):
   ...
   ...
   (Some validation logic longer than 10 lines)
   ...
   ...

some_condition.__name__ = "Error message" # When define it as naked function
some_condition = Schema(some_condition, error="Error message") # When define it as object

Predicate make it possible to define a functional schema as one def statement.

@Predicate(error="Error message")
def some_condition(x):
   ...
   ...

I feel later style is more readable (in the same reason of @getter defining properties).

[skorokithakis]

Thank you for your contribution! Hmm, this looks useful, but I don't know if the verbose way is currently so big a pain to justify including this predicate in the library. On the other hand, it can't hurt, but it does make the library a bit more complicated. You're literally replacing this:

some_condition = Schema(some_condition, error="Error message")

with this:

@Predicate(error="Error message")

correct? The latter is more readable, true, but I'm not sure that's not enough justification to include this. Does anyone else want to chime in?

[codecov-io]

Codecov Report

Merging #146 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #146   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         208    212    +4     
=====================================
+ Hits          208    212    +4

Impacted Files	Coverage Δ
schema.py	100% <100%> (ø)	:arrow_up:

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