support alignment attribute of components

Components in .glyphs source files may contain an `alignment` attribute.

There's no mention about this in the [GlyphsFileFormat.md](https://github.com/schriftgestalt/GlyphsSDK/blob/master/GlyphsFileFormat.md) specification.

In my observations, it can take a value of `1` or `-1`; and I suppose that when the attribute is missing the value is `0`?

I am not sure what these values do, and how a user can set them from the interface.
I'd be nice if @schriftgestalt could shed some light on this, so we can add support in `glyphsLib`.

Start by reading GlyphsFileFormat.md and the glyphsLib handling of components in .glyphs source files. Confirm what alignment values mean and how users set them through the interface, then define the expected support and update the specification and library accordingly.

Written by the indexing model from the issue text.