os.outputof: add a second argument to select which stream to output

[noresources]

local o, e = os.outputof(cmd, streams)

Where streams could be one of
- "output" Only return standard output stream content
- "error" Only return standard error stream content
- "both" (default) Return both streams content

How does this PR change Premake's behavior?
Allow to filter standard output or error stream content from os.outputof() return value.
Ignoring error output is important when the invoked command may return an exit code 0 (success) but output some content on the error stream which will pollute the text result.

Are there any breaking changes? Will any existing behavior change?
The default behavior follow the previous one (report both streams)

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
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