Add conditional breakpoints

[dylandreimerink]

This issue proposes to add conditional breakpoints, which would allow a user to specify a expression(#16) to a breakpoint. Only when this expression evaluates to true would the breakpoint be triggered. This feature is very handy if you want to debug a specific condition which only occurs on for example the 200'th iteration of a loop, by being able to trigger on i == 200 you don't have to break and continue 200 times manually.

This issue depends on having the expression logic described in issue #16 in place, and is blocked until that issue is taken care of.