Add ParallelMap function

[aswinkarthik]

The API and behaviour is same as godash.Map. Same validations apply. Just that the mapperFn has to be executed on all elements in Go routines and the output is collected and returned. godash.ParallelMap is still a blocking function.
Insipired from java streams parallelMap()

[dineshba]

Is it one go routine per element ? Can we have a way to control the number of routines(as function parameter) ?

[aswinkarthik]

Lets go with one go-routine per element and no need for a way to control the parallelism.