Boston’s theatre space crunch may be eased a bit next year, if all goes according to plan for a local troupe. The Huntington Theatre Company announced yesterday that it is halfway to its fundraising goal to build two new theatres in the Boston Center for the Arts complex. One of the new theatres would seat 360, the other 200, making them a welcome addition to the Boston scene, which has lacked adequate small and mid-sized venues for years.