Implement lenientIf option (resolve #265; default off)

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

src/liquid-options.ts

@@ -19,6 +19,8 @@ export interface LiquidOptions {
   strictFilters?: boolean;
   /** Whether or not to assert variable existence.  If set to `false`, undefined variables will be rendered as empty string.  Otherwise, undefined variables will cause an exception. Defaults to `false`. */
   strictVariables?: boolean;
+  /** Modifies the behavior of `strictVariables`. If set, a single undefined variable will *not* cause an exception in the context of the `if` tag and the `default` filter. Instead, it will evaluate to `false` and `null`, respectively. Irrelevant if `strictVariables` is not set. Defaults to `false`. **/
+  lenientIf?: boolean;
   /** Strip blank characters (including ` `, `\t`, and `\r`) from the right of tags (`{% %}`) until `\n` (inclusive). Defaults to `false`. */
   trimTagRight?: boolean;
   /** Similar to `trimTagRight`, whereas the `\n` is exclusive. Defaults to `false`. See Whitespace Control for details. */
@@ -56,6 +58,7 @@ export interface NormalizedFullOptions extends NormalizedOptions {
   dynamicPartials: boolean;
   strictFilters: boolean;
   strictVariables: boolean;
+  lenientIf: boolean;
   trimTagRight: boolean;
   trimTagLeft: boolean;
   trimOutputRight: boolean;
@@ -85,6 +88,7 @@ export const defaultOptions: NormalizedFullOptions = {
   outputDelimiterRight: '}}',
   strictFilters: false,
   strictVariables: false,
+  lenientIf: false,
   globals: {}
 }