#39: modified JumpToHeader to accept counts and avoid polluting search history

[muru]

Notes:

• I had to add <C-u> to the normal mode mappings as well before [[ started accepting a count properly, following suggestions from http://vi.stackexchange.com/q/4366/205
• I used :silent! execute since:
  1. @romainl used it
  2. The help for :silent uses :silent exe "normal ...", so presumably that's the preferred way
  3. And :/, :? seems to work well, so calling normal seemed unnecessary.

I'm not generally used to Github pull requests (messed up the only one I tried before), so if I should have done something differently (use a branch or something), my apologies.

[gabrielelana]

@murukeshm Yes, a pull request from a feature branch would have been better, but great work! That's a nice feature, thank you for your contribution

P.S. If you want, this is a nice and detailed tutorial about Github, branches and pull requests