Add the feature to customize the location of the project files.

[xuhdev]

I added a new option g:atp_ProjectFileLocation. I also updated the document so you can see my explanation there, though I'm not sure whether I put the doc in the correct place. Also, feel free to update my wording as I sometimes use words unnaturally.

[coot]

There are few think which needs to be fixed. The b:atp_ProjectDir will be set to wrong location, and in does not work when there is no project.vim file. This will need a bit more testing/time to get upstream. Thanks for the patch though I'll work on in on a new branch: https://github.com/coot/atp_vim/tree/xuhdev-project-file-location

[xuhdev]

Thanks!