Chop up GSUB subtables into their own set of files

[RoelN]

Fixes #101, if this is you had this directory/naming in mind :-)

[Pomax]

This is great! But while both GSUB and GPOS have lookup types, they're wildly different, they're not really "shared", so let's create the lookups dir inside the advanced dir instead of under shared, with dedicated dirs for gsub and gpos:

--src
 |--opentype
 | |--advanced
 | | |--shared
 | | |--lookups
 | | | |--gsub
 | | | | |--gsub-lookup.js
 | | | | |--lookup-type-1.js
 | | | | |--lookup-type-2.js
 | | | | |--...
 | | | |--gpos
 | | | | |--gpos-lookup.js
 | | | | |--lookup-type-1.js
 | | | | |--lookup-type-2.js
 | | | |

[Pomax]

let's change the file paths just a little, see the comment in the "conversation" tab.

[Pomax]

It also looks like gsub.js contains the SubstLookupRecord class required by both lookup types 5 and 6, but both modules forget to import that class.

[RoelN]

Thanks for the review. Good catch on SubstLookupRecord! All things should've been addressed with the last few commits.

[RoelN]

In preparation of #45 (comment), I set up placeholder code for GPOS as well, so we don't have that in the old format.

[Pomax]

nice!