Use Tasker with "OK Google" on Nexus 5
Here is a video tutorial on how to get it setup.  

https://www.youtube.com/watch?v=-6Woh20ullI

Explanation of Video

This requires the Xposed Framework and the Google Now API for it. You will need to be rooted to install the Xposed Framework.

First you need to do this:


Open Google Now API
Click plugin (example AutoVoice)
Check "Enable" for Google Now integration


In Tasker you will need to setup something similar to task for task:


Create a new Action
Create If (for action category)
For condition select (or type if you cant find) avsource
Make sure condition is set to matches
type in googlenow for what the condition matches
Create whatever action you want for the If (need to be familiar with Tasker)
Create End If