Fuzzy matching for .npmrc profiles

This allows switching to a specific .npmrc profile just by typing its first few unique characters, for example:

```shell
$ npmrc def
Removing old .npmrc (/Users/conrad/.npmrcs/work)
Activating .npmrc 'default'
```

If there are multiple profiles that match, `npmrc` lists them:

```shell
$ npmrc test
'test' is ambiguous. Found multiple matching npmrcs:

  test1
  test2
```