Allow setting temporary directory path

A potential use case for vipe is editing an encrypted file; `decrypt | vipe | encrypt`

Currently vipe always saves the temporary file to `/tmp/` which on many systems is a tmpfs. However, tmpfs can be swapped thus resulting in sensitive data being written to disk. To solve this we can create a ramfs which cannot be swapped, but we would need to be able to configure vipe to use this directory. I imagine we could pass in a temporary directory something like this:

```
decrypt | vipe -t /myramdisk | encrypt
```

Start at vipe's command-line entry point and trace how it creates the temporary file; the issue names no file or test. Check how options are parsed, then verify that `vipe -t /myramdisk` uses the supplied directory while preserving the existing default behavior.

Written by the indexing model from the issue text.