Add git_add_force rule

This adds `--force` to `git add` when needed. For example:

    $ git add dist/*.js
    The following paths are ignored by one of your .gitignore files:
    dist/app.js
    dist/background.js
    dist/options.js
    Use -f if you really want to add them.
    $ fuck
    git add --force dist/app.js dist/background.js dist/options.js [enter/↑/↓/ctrl+c]
    $

README.md

@@ -166,6 +166,7 @@ using the matched rule and runs it. Rules enabled by default are as follows:
 * `fix_file` – opens a file with an error in your `$EDITOR`;
 * `gem_unknown_command` – fixes wrong `gem` commands;
 * `git_add` – fixes *"pathspec 'foo' did not match any file(s) known to git."*;
+* `git_add_force` &ndash; adds `--force` to `git add <pathspec>...` when paths are .gitignore'd;
 * `git_bisect_usage` &ndash; fixes `git bisect strt`, `git bisect goood`, `git bisect rset`, etc. when bisecting;
 * `git_branch_delete` &ndash; changes `git branch -d` to `git branch -D`;
 * `git_branch_exists` &ndash; offers `git branch -d foo`, `git branch -D foo` or `git checkout foo` when creating a branch that already exists;