Minimal implementation of search

[skissane]

This just does the bare minimum to add a search function (#10)

A lot of things are missing:

• only exact match, no support for inexact matches (substring, glob, regex, etc)
• should -d option be ignored when searching?
• silently ignored with interactive mode; maybe it should display an error for unsupported combination of options?
• exit status could communicate whether match is found or not (like grep)

But it is a start.