Add ability to change link opening behaviour by usage of Shift

[rymndhng]

In Vimium, you can change the behaviour of link opening by holding down shift when typing in the hint. So for example, if there's a hint FD, I would expect typing:

f d to open the link in the current tab
F D would open the link in a new tab

When there is ambiguity I would expect the presence of a shift press on the last character to open in a new tab.

[gggritso]

Well, F d will open the link in a new tab. Why hold shift the whole time?

[rymndhng]

Ah, the idea is you don't need to hold shift the entire time. You only need to hold shift for the last character (in the case of pages with many links)

[gggritso]

That's reasonable. I’ll schedule this and put it into the next patch version.