speed up when change directory in zsh

[iamxiaobo]

This pull request resolve two questions:

1. For example, my project path is /home/user/work/nvm, in this project directory there is a .nvmrc file with v4.4.7, and my nvm default is v6.7.0。if i enter /home/user/work/nvm, nvm change node to v4.4.7 which is reasonable. but if i enter /home/user/work/nvm/public, nvm use default invoked and now the node become v6.7.0. use the new code, when enter /home/user/work/nvm/public, nvm not only check if .nvmrc exist in this directory, bus also check parents directory which can solve my problem.
2. In current version of nvm, if my node version is v6.7.0 and the directory i enterd have .nvmrc file with content of v6.7.0, nvm also invoke nvm use and cost some time, it's meanless.

[ljharb]

This code can get a lot simpler if you use nvm_find_nvmrc which will print out the first .nvmrc it finds - it's the same code nvm uses internally.

[iamxiaobo]

OK, I have updated my code by your suggestion

[ljharb]

this won't work because the .nvmrc file could contain 1 or v1.0 or v1.0.0 or foo (where "foo" is an alias) - you'd need $(nvm version "$(cat "${nvmrc_path}")")

[ljharb]

LGTM! mind rebasing this down to a commit?

[iamxiaobo]

@ljharb , thanks for your review