Allow sharing traces with -share flag

[EmeraldShift]

Re-introduce a PR (original: #132) to add a -share flag when both MAGIC_TRACE_SHARE_DOC and MAGIC_TRACE_SHARE_COMMAND are provided. It's available as an alternative to -serve, and both cannot be provided in the same invocation.

For example, with

MAGIC_TRACE_SHARE_DOC=" Share the trace." MAGIC_TRACE_SHARE_COMMAND=/usr/bin/echo magic-trace run -help

I see the flag in the CLI help:

  [-share]                   . Share the trace.

and with a real program to run, the command is invoked, and I see the trace file name echoed into my terminal.

[cgaebel]

lgtm, ty