Update inputSourceMap docs/types

* `inputSourceMap` is checked for truthiness in `utils` and passed to `source-map`'s `SourceMapConsumer`, which accepts either a JSON string or an object - updated types to allow an object
* Discussion in #501  seems to imply that this does not necessarily need to be a sourcemap generated by `recast`, despite the language that was previously

Author: flotwig

lib/options.ts

@@ -74,12 +74,12 @@ export interface Options extends DeprecatedOptions {
   sourceRoot?: string | null;
 
   /**
-   * If you provide a source map that was generated from a previous call
-   * to recast.print as options.inputSourceMap, the old source map will be
-   * composed with the new source map.
+   * If a source map is provided here, it source map will be composed 
+   * with the new source map. The source map can be provided as a
+   * JSON string or as an object.
    * @default null
    */
-  inputSourceMap?: string | null;
+  inputSourceMap?: string | any | null;
 
   /**
    * If you want esprima to generate .range information (recast only uses