option to replace the command marker "$>" with an alternative for single line commands

[prednaz]

This adds a command line option to replace the command marker "$>" with an alternative. Only single line commands like

-- $> not True

are affected.

Mutliline commands like

{- $>
not True
not False
<$ -}

are not affected.

I needed this for code that contains Haskell Language Server style command comments.

But --eval-mark ">>>" does not achieve perfect interoperability because the Haskell Language Server's multiline commands are still not understood.

I admit that Haskell Language Server's evaluation feature does not serve the exact same purpose as ghcid's. But sometimes, especially in an educational context, it is used exactly like ghcid's evaluation feature.

Thanks for the pull request!

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If you care, my PR preferences are at https://github.com/ndmitchell/neil#contributions, but they're all guidelines, and I'm not too fussy - you don't have to read them.