Add impl_file_name_tmpl

[doroginin]

Added new flag impl_file_name_tmpl - template for generating implementations filename, default template: {{ if .MethodName }}{{ .MethodName }}{{ else }}{{ .ServiceName }}{{ end }}.

Changed default template name in mode impl_service_sub_dir=false from {{.ServiceName}}{{if .MethodName}}.{{.MethodName}}{{end}} to {{ if .MethodName }}{{ .MethodName }}{{ else }}{{ .ServiceName }}{{ end }}

example of usage:

protoc --gofast_out=plugins=grpc:. --goclay_out=impl_file_name_tmpl="{{.ServiceName}}{{if .MethodName}}.{{.MethodName}}{{end}}",impl=true,impl_path=./app:. my.proto