.bashrc: define `npm` wrapper which strips background colours

### Before

<img alt="`npm install` before" src="https://cloud.githubusercontent.com/assets/210406/2808070/75f555ca-cd0d-11e3-8c45-d3ee50f3eb1c.png" width="623" height="601" />

### After

<img alt="`npm install` after" src="https://cloud.githubusercontent.com/assets/210406/2808072/7a05a07a-cd0d-11e3-9bd5-32de7670c004.png" width="623" height="601" />