Fix issues with 2.0.0

• Allow overridable members to be overridden (!)
• Return data from submodule require calls in case it has been overridden with a synchronous method

`modulePrototype` param; `require` now overrideable

• Add a new third parameter to get-smart function (and .fetch, .ajax, .all, and js reviver function) to serve as a prototype for module in .js files, providing access to selected variables
• Expose getSmart.require which is the function called by calls to require() made from submodules
• Make the getSmart function constructible, extract getSmart.fetch, and add references to all existing props to getSmart.prototype
• Throw error for modules that call require() but don’t appear to assign to module.exports as required
• Throw error for modules that call require() and assign to module.exports more than once
• Allow (but ignore) substring between ; and . in url
• Bumping to new major version 2.0.0 in recognition of breaking change that a file name without an extension now gets the default extension (.js). A file without an extension can still be loaded by specifiying the null extension (.) explicitly.

Now supports `require`; filename and extension defaults; expose `revivers`

• .js is now the default extension for filenames with no extension
• index.js is now the default filename when no filename is given in the URL (if path ends in ., .., or /)
• An asynchronous implementation of require is now available for use inside .js modules
• Expose getSmart.revivers, a dictionary of file processors which the application developer can augment or override

unpkg support

Update build.sh to create umd folder for unpkg.com CDN support for this and all future versions. See revised installation snippet above. (get-smart.github.io will no longer be updated with new versions, although version 1.0.2 will remain there.)

Resolve IE-11 issue

Avoid using Object.assign for IE-11 compatibility.