Implement lenientIf option (resolve #265; default off)

See description of the option in liquid-options.ts.

src/template/value.ts

@@ -4,6 +4,7 @@ import { FilterMap } from '../template/filter/filter-map'
 import { Filter } from './filter/filter'
 import { Context } from '../context/context'
 import { ValueToken } from '../tokens/value-token'
+import { assert } from '../util/assert'
 
 export class Value {
   public readonly filters: Filter[] = []
@@ -18,7 +19,10 @@ export class Value {
     this.filters = tokenizer.readFilters().map(({ name, args }) => new Filter(name, this.filterMap.get(name), args))
   }
   public * value (ctx: Context) {
-    let val = yield evalToken(this.initial, ctx)
+    assert(ctx, () => 'unable to evaluate: context not defined')
+    const lenient = ctx.opts.lenientIf && this.filters.length > 0 && this.filters[0].name == "default"
+    
+    let val = yield evalToken(this.initial, ctx, lenient)
     for (const filter of this.filters) {
       val = yield filter.render(val, ctx)
     }