Update to sile 0.14.16 and make silex optional

[Omikhleia]

The re-implementation of some classes, packages, etc. provided by sile·x no longer takes over that of SILE. Users have to enforce the use of these replacements explicitly:

• require("silex") to enforce all of sile·x
• require("silex.override") to only enforce the replacements

The resilient.sile module already does the first when the resilient classes are used, so the latter get all of the sile·x features enabled.

Other classes and modules are no longer affected without the user explicit will and intent, so there is now far less risks of breaking anyone's workflow when installing sile·x

Additionally, the re-implemented sile·x typesetter is re-aligned on SILE 0.14.16, meaning it benefits from its new features:

• Soft hyphen support
• Speaker change in dialogues

Obviously, all its "special" experimental features are still there.

[Omikhleia]

Nah outputters break. Loaded before our override hack likely. :cry:

0.4.0 removed from luarocks and marked as pre-release.