add customUserAgent to ConfigurationOptions (#1459)
add customUserAgent to ConfigurationOptions (#1459)

* add customUserAgent to ConfigurationOptions

* clarify user agent behavior

GlideMe/aws-sdk-js,guymguym/aws-sdk-js,chrisradek/aws-sdk-js,chrisradek/aws-sdk-js,jeskew/aws-sdk-js,jeskew/aws-sdk-js,jeskew/aws-sdk-js,GlideMe/aws-sdk-js,GlideMe/aws-sdk-js,GlideMe/aws-sdk-js,chrisradek/aws-sdk-js,guymguym/aws-sdk-js,guymguym/aws-sdk-js,aws/aws-sdk-js,chrisradek/aws-sdk-js,aws/aws-sdk-js,aws/aws-sdk-js,jeskew/aws-sdk-js,aws/aws-sdk-js,guymguym/aws-sdk-js