Add .ptignore for ignoring patterns. Searched in path as well as home directory

Added a default file ".ptignore" which can be parsed by pt for ignore patterns.
This can be searched in the paths (as part of vcsignores) as well as user can have one in his home directory for global rules that he wants to follow across projects etc.
There is also an option --noptignore to not use the global/home directory .ptignore file if needed.
For the local .ptignore, custom vcsignore can be specified as already to ignore this if needed.

I apologize if there are any mistakes in coding style or the code itself. This is my first stab at golang with nothing but your code as reference. Willing to spend more time in it though :) Please let me know if I need to update anything.