GF shell: change parse & linearize to obtain useful results from p|l and l|p in more cases

These changes are inspired by the gf -cshell implementation of these commands.

The output of the linearize command has been changed to remove superfluous
blank lines and commas, and deliver the result as a list of strings instead of
a single multi-line string. This makes it possible to use -all and pipe the
results to the parse command. This also means that with -treebank -all,
the language tag will be repeated for each result from the same language.

The parse command, when trying to parse with more than one language, would
"forget" other results after a failed parse, and thus not send all
successful parses through the pipe. For example, if English is not the first
language in the grammar,

    p "hello" | l

would output nothing, instead of translations of "hello" to all languages,
forcing the user to write

   p -lang=Eng "hello" | l

instead, to get the expected result. The cause of this behaviour was in the
function fromParse, which was rather messy, so I assume it is not intentional,
but the result of a programming mistake at some point.

The fromParse function has now been refactored from a big recursive function
into 

    fromParse opts = foldr (joinPiped . fromParse1 opts) void

where the helper functions fromParse1 deals with a single parse result and
joinPiped combines multiple parse results.

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