Make Search-on-open optional

Adds a variable, `g:TurboMarkSearchOnOpen`, to control entering search mode when the quickfix window is opened.

I felt that inserting the `/c` is so different from any other quickfix plugin that it makes using TurboMark very confusing at first. I understand the idea of being able to search instantly, but ideally there would be a way to support both search and using other forms of navigation, such as how [ctrlp](https://github.com/kien/ctrlp.vim) works.

In this version, the automatic search defaults to off and can be enabled by a user who has a desire to do so.

Fixes #1