Ns index summary

Forked from [this PR](https://github.com/weavejester/codox/pull/25/files).

    codox.utils=> (summary "Test")
    "Test"
    codox.utils=> (summary "Test\n\nX")
    "Test"
    codox.utils=> (summary "Test\n\n\nX")
    "Test"
    codox.utils=> (summary "\nTest\n\n\nX")
    "Test"
    codox.utils=> (summary "Test\fX")
    "Test"