added option to always display tag name, so that we can pipe its output ...

Hey, 

I wanted to filter `dalvikvm` logs from `pidcat` output. So, it would be nice to have an option to always display tagname, so that we can pipe its output to `grep` and filter certain tags.

```bash
./pidcat.py -l D  --always-display-tags com.trhura.android.somepackage | grep -v -e "dalvikvm"
```