implemented an accessser of #asString()

[bigen1925]

ISSUE

https://github.com/evanshortiss/env-var/issues/146

Description

Basically I followed this StackOverflow post, but replaced expressions of unicodes of '\xhh' with '\uhhhh' because they are not standard in EcmaScript. refs) https://stackoverflow.com/questions/29028756/javascript-unicode-regex-range-out-of-order-in-character-class

[coveralls]

Coverage remained the same at 100.0% when pulling 5a66efc6aa1cadda8b8c746884d6a665d6056f51 on bigen1925:feature/asString into 3f5689ed13b307e4389b7dcf642bac55738e534c on evanshortiss:master.

[evanshortiss]

Thanks @bigen1925!