Add properties for AlignExprs, make content->text

Items and Metadata could specify "content", which is not the content AlignmentExpression. This has been changed to "text", as "content" is now the value of the content alignment expression attribute (similarly, "alignment" and "segmentation" are now accessible from Items and Tiers as properties, though they may to the internal attributes dictionary).

I also separated the code for attribute defaults and inheritance to its own XigtAttributeMixin, which also handles the setting of id and type values.
Make get_[alignexpr]_ids() more robust.
Add latex exporter.
Add top-level import and export scripts, latex out

This commit improves the latex formatter, though it is still pretty limited.

The top-level scripts import.py and export.py act as interfaces to the modules in importers/ and exporters/. Not only is it more convenient, but avoids import problems (such as the Xigt module 'toolbox' shadowing the 'toolbox' package elsewhere in the system).
Make latex export more robust.

Also, instead of a replacements file, the -c --config option allows for a JSON-formatted config file, which can have more than just replacements. Currently the latex export allows for item-level and tier-level replacements, and they now have regex matches for the tier type so they can be selectively applied.
Add select() function on Xigt objects.
Update toolbox importer to current ver.
Update LaTeX exporter.

xigt_export() function now takes an output path, rather than file handle, to account for non-file paths like directories

sub() function in latex.py is moved to util.py, so it's useful for other exporters

made 'tier_types' an entry in the config file, resorting to the defaults in latex.py, rather than only using the default values.
Add itsdb exporter.