cache ignore files

[albfan]

The naive approach for caching ignore files is commented

https://github.com/albfan/nerdtree-git-plugin/blob/master/nerdtree_plugin/git_status.vim#L107

See if this is really a bottle neck caching based on content using system sha1sum or incorporating sha1 vim implementation

https://github.com/vim-scripts/sha1.vim

[albfan]

not strictly blocking but #10 would perform better with this implemented