Fix treatment of slashes (path separator) by history command

[rakitzis]

This change fixes the history pruning treatment of a slash character in the history followed by the name of the history command.

The purpose of pruning is to avoid recursion in the history rewind, so history lines with a history command in them are pruned away in the search.

With the history command named as "-", previously a history line like:

    find / --option

would have been pruned (too aggressively). The change trims whitespace after first considering the case of whitespace separating a slash and the name of the history command.

A few basic unit tests for the history command are also added with this commit.

[rakitzis]

Fixes #104

[gitboy]

thanks for fixing this!