refactor: create new collector folder

Create a new `collector` folder to store the code for the different
collectors. Move the existing f2b and textfile collectors to this folder.
Minor refactors to the f2b collector to better match the code style of the
newer textfile collector.

src/collector/textfile/writer.go

@@ -0,0 +1,20 @@
+package textfile
+
+import (
+	"log"
+	"net/http"
+)
+
+func (c *Collector) WriteTextFileMetrics(w http.ResponseWriter, r *http.Request) {
+	if !c.enabled {
+		return
+	}
+
+	for _, f := range c.fileMap {
+		_, err := w.Write(f.fileContents)
+		if err != nil {
+			c.appendErrorForPath(f.fileName)
+			log.Printf("error writing file contents to response writer '%s': %v", f.fileName, err)
+		}
+	}
+}