Handle falsy values (#4)

* Handle falsey values

Previously, if a falsey value was passed (e.g. `null` or `undefined`), an error would occur. This change returns an empty string instead.

Modify changes from budokans's PR to limit conversion of "--" to ' - …

…' to only occasions when the original character was an em dash.

Revert mocha package version so unit testing will work on older versi…

…ons of Node.js.

Fix typings.

Update README.md and CHANGELOG.md. Remove tabs that somehow crept in …

…to replace spaces. Promote to version 1.0.0.

Release v0.1.8.

Release v0.1.7.

Release v0.1.6.

Release v0.1.5.

Release v0.1.4.