change match/split regex to only match require()

statments when the line starts with whitespace or require().
This allows require() statements to be temporarily remove
with single line comments and ignores require() statements
that are part of a normal javascript statement.

fixes #7

Not sure if this steps on the toes of any of the use cases for grunt-neuter... but it does match the regex of the neuter implementation found here: https://github.com/wycats/rake-pipeline-web-filters/blob/master/lib/rake-pipeline-web-filters/neuter_filter.rb#L30