Added detection of default browser

Hi,

thanks for creating this project! It's pretty nice and does the job well. I thought it would also be useful to find what is the default browser of the user, so I added a few lines to detect that.

 I hope the code is straightforward. I went with separate variable for default browser since it probably makes sense to have it that way, but foremost for implementation results - likely the default browser shares the path with some named-browser which would then make one of them discarded due to duplicated path.

This is the output on my machine now; Win7 x64. It finds even chrome canary and firefox nightly, nice :+1:)

```
chrome 38.0.2125.104
  @ C:\Program Files (x86)\Google\Chrome\Application\chrome.exe

ff 33.0.0.5397
  @ C:\Program Files (x86)\Mozilla Firefox\firefox.exe

ie 9.0.8112.16575
  @ C:\Program Files\Internet Explorer\iexplore.exe

ie 9.0.8112.16575
  @ C:\Program Files (x86)\Internet Explorer\iexplore.exe

opera 25.0.1614.63
  @ C:\Program Files (x86)\Opera\Launcher.exe

ff 36.0.0.5400
  @ C:\Program Files (x86)\Nightly\firefox.exe

chrome 40.0.2202.3
  @ C:\Users\<username>\AppData\Local\Google\Chrome SxS\Application\chrome.exe

default 33.0.0.5397
  @ C:\Program Files (x86)\Mozilla Firefox\firefox.exe
```