Support -M and -m options equivalent to perl's for all ops with snippets

A very useful convenience, for example:

    recs-grep -MList::MoreUtils=any -e 'any { {{Name}} eq $_ } qw(foo bar bat)'

The symantics of -M/-m match the documented behaviour in perlrun and the
produced Perl syntax.

Getopt bundling is required for -MFoo=bar to work, otherwise an error is
thrown regarding unknown option "-MFoo".

Note that, for the time being, use of -M requires explicit use of -e/-E.
Don't require explicit use of -e/-E when -M/-m is used
Test new -M functionality, by way of xform

List::Util is core since 5.7.3.
Document -M/-m in executor and provide example in grep

Executor's help is only used by xform, for now.
Include help for executor options for operations which accept them

xform includes it already.
switch fromxml from XML::Simple to XML::Twig

Preserves document order (making UTs pass in perl 5.18)
Handles large documents
Fixes bug where ForceArray was always set to 1
Fixes bug where 'element' entries weren't always added when multiple --element were specified
Merge pull request #23 from tsibley/dash-M

Support -M and -m options equivalent to perl's for all ops with snippets
deduplicate --elements; add argument handling test cases; minor style/documentation changes
force all nested elements into arrays
add XML related todos
Merge pull request #25 from elindsey/twig

switch fromxml from XML::Simple to XML::Twig