Add trackCustomEvent [Fixes #34]

[samajammin]

Solves #34

[d4h0]

@samajammin: Shouldn't this be defined in gatsby-browser.js?

I'm pretty sure, calling trackCustomEvent outside the browser will fail because window doesn't exist.

[Hahlh]

@d4h0 a simple window check, e.g: if (typeof window !== 'undefined') { // do something }, should suffice to address that if necessary.