Feature Request: Ability to `rename` and`chmod` resultant file

## what
* I wish that we could specify the octal mode of the file download from the github release
* I wish that we could specify the destination name

## why
* So that I can set the execution bit (E.g. `0755`) without calling `chmod` explicitly
* Extremely common usage pattern is to download binary releases of apps (e.g. `fetch`) and then run them

## example interface
```
fetch --repo https://github.com/gruntwork-io/fetch \
--tag=v0.2.0 \
--release-asset=fetch_darwin_amd64 \
--asset-name=fetch \
--asset-file-mode=0755 \
/usr/local/bin
```