Prevent keyboard shortcuts from firing when modifier keys are used

Hello,

I have a bad habit of hitting Ctrl-N to open new page/tab in the middle of reading an RSS article. Currently, the N part of the shortcut will be caught by the Miniflux keyboard shortcuts function and cause it to go to next article when I'm not necessarily done with this one yet.

I've made a change to ignore the keyboard shortcuts when Alt, Shift, or Ctrl are used with the shortcut key. I think that should be fine - shortcuts are currently specified as "gu" or "n" and this continues to work.

(I've also tried to filter for Windows/Meta/Super key, but browser support looks iffy, I couldn't get it to work in my Firefox 32 on Windows. But just the other three should be an improvement. I've left the code for e.metaKey as well in case that works in the future.)

Please take a look and merge if you like / agree with the idea.

Thanks for the work on Miniflux!

--Jarek