Use backticks to escape regular expressions

[lukasmalkmus]

I modified the code to use backticks and added an appropriate regex in the tests which failes when using upstream/master as described in #32. This PR fixes #32.

[lukasmalkmus]

While the provided regex must still be escaped (in order to get compiled by the protoc compiler), the compiled files have their regex in backticks without the escaped backslashes. Pull this PR, run make test and take a look at the generated validator files:

var _regex_ValidatorMessage3_SomeStringUnescaped = regexp.MustCompile(`[\p{L}\p{N}]({\p{L}\p{N}_- ]{0,28}[\p{L}\p{N}])?.`)

[mwitkow]

It's unfortunate that the protoc string still needs to be escaped. But still much better than it used to be :) Thanks!