Fix git stash pop with local changes

When there are local changes to a file, and a git stash is popped that
contains other changes to that same file, git fails as follows:

    $ git stash pop
    error: Your local changes to the following files would be overwritten by merge:
            src/index.js
    Please commit your changes or stash them before you merge.
    Aborting
    $

This change adds a rule that corrects this problem as suggested [here]:

    $ git stash pop
    error: Your local changes to the following files would be overwritten by merge:
            src/index.js
    Please commit your changes or stash them before you merge.
    Aborting
    $ fuck
    git add . && git stash pop && git reset . [enter/↑/↓/ctrl+c]
    Auto-merging src/index.js
    On branch flow
    Changes to be committed:
      (use "git reset HEAD <file>..." to unstage)

            modified:   src/index.js

    Changes not staged for commit:
      (use "git add <file>..." to update what will be committed)
      (use "git checkout -- <file>..." to discard changes in working directory)

            modified:   src/index.js

    Dropped refs/stash@{0} (f94776d484c4278997ac6837a7b138b9b9cdead1)
    Unstaged changes after reset:
    M        src/index.js
    $

[here]: https://stackoverflow.com/questions/15126463/how-do-i-merge-local-modifications-with-a-git-stash-without-an-extra-commit/15126489#15126489

README.md

@@ -187,6 +187,7 @@ using the matched rule and runs it. Rules enabled by default are as follows:
 * `git_rebase_merge_dir` – offers `git rebase (--continue | --abort | --skip)` or removing the `.git/rebase-merge` dir when a rebase is in progress;
 * `git_remote_seturl_add` – runs `git remote add` when `git remote set_url` on nonexistant remote;
 * `git_stash` – stashes you local modifications before rebasing or switching branch;
+* `git_stash_pop` – adds your local modifications before popping stash, then resets;
 * `git_two_dashes` – adds a missing dash to commands like `git commit -amend` or `git rebase -continue`;
 * `go_run` – appends `.go` extension when compiling/running Go programs;
 * `gradle_no_task` – fixes not found or ambiguous `gradle` task;