refactor(compiler): resolve output before args (#17)

Problem: presets that need the output path in their args function
(markdown, github) had to recompute it inline, duplicating the same
gsub expression already in the output field.

Solution: resolve output_file first in M.compile, then extend ctx with
output = output_file into a resolved_ctx before evaluating args and cwd.
Presets can now reference ctx.output directly. Add output? to the
preview.Context type annotation.

spec/presets_spec.lua

@@ -150,6 +150,7 @@ describe('presets', function()
       file = '/tmp/document.md',
       root = '/tmp',
       ft = 'markdown',
+      output = '/tmp/document.html',
     }
 
     it('has ft', function()
@@ -233,6 +234,7 @@ describe('presets', function()
       file = '/tmp/document.md',
       root = '/tmp',
       ft = 'markdown',
+      output = '/tmp/document.html',
     }
 
     it('has ft', function()