Rewrite to generalize mappings (fix #13) and global cache (fix #50)

* Support mapping defined by `Map` or `WeakMap`
* All function mappings are now automatically dynamic;
  no need to wrap in a second layer of function.
* Can force old static behavior (for efficiency)
  via `svgtiler.static` wrapper, cached at `Mapping` level.
* Function mappings get `context` second argument in addition to `this`.
* `Context` object now shared throughout each drawing
  (so you could store extra drawing-specific data there).
* API change: `Context` no longer has `symbols`, `symbol`, `data`,
  `filename`, or `subname` attributes.  For the latter three,
  use `drawing.data`, `drawing.filename`, or `drawing.subname`.
* API change: No more `Symbol`, `StaticSymbol`, or `DynamicSymbol` class
  (in particular, there was a name conflict with JS's `Symbol`)
* API change: `Input` constructor supports direct loading of data
  (for easy manual creation) or immediate parsing of file data,
  as well as option setting.  This will help with #34.
* New `Tile` class represents an expanded (and possibly parsed)
  tile `<symbol>` (roughly what `Symbol` was before).
* Tile caching/deduplication now within the renderer, not by `Symbol`
  (fix #50), enabling re-use when multiple symbols expand to the same SVG.
* Fix: Don't fill in `<image>` `width`/`height` when both already set.
* Fix: Use `hasOwnProperty` and `Set`s to avoid accidentally matching
  e.g. `toString`.

README.md

@@ -180,6 +180,7 @@ In any case, `mapping` should be an expression evaluating to either
 2. a *function* in one argument, a symbol name (string).
    (This feature allows you to parse symbol names how you want; or check an
     object for a matching key but use a default value otherwise; etc.).
+xxx Map
 
 The object or function should map a symbol name to either
 
@@ -197,6 +198,8 @@ The object or function should map a symbol name to either
 4. a filename with `.png`, `.jpg`, `.jpeg`, or `.gif` extension containing an
    image (which will get [processed](#image-processing) as an `<image>`), or
 5. a function returning one of the above.
+xxx undefined means 'symbol not defined by this mapping, go to next mapping'
+xxx empty string means 'blank 0x0 symbol'
 
 In the last case, the function is called *for each occurrence of the symbol*,
 and has `this` bound to a manufactured `Context` object, giving you access to