build: Support debug builds; build refactoring (#279)

As mentioned in #271, this adds support for making debug builds.

* Use yargs to improve options-handling in the build script.
* Simplify the npm scripts back to building/cleaning the extension for all browsers.
* Remove the "build:all:quick" script in favour of just calling the build script directly on the CLI.
* Remove the npm clean scripts; most of this is now done in the build script, which now has a clean mode.
* Support debug builds (in which console.timeStamp() calls are made).
* Update the README accordingly.

README.md

@@ -112,14 +112,9 @@ You can build and run the current code locally as follows.
 
 2.  Ensure you have all the required build tools with `npm install` (you will need [Node.js](https://nodejs.org/)).
 
-3.  Run the build script to build one or all of the extensions:
+3.  Run the tests and build script to create versions of the extension for all browsers with `npm run build:all`.
 
-    -   `npm run build:firefox`
-    -   `npm run build:chrome`
-    -   `npm run build:opera`
-    -   `npm run build:all`
-
-    The built versions of the extension are placed in the `build/<browser>/` directories and ZIP files for each will be created in the root of the checked-out repository.
+    The built versions of the extension are placed in `build/<browser>/` directories and ZIP files for each will be created in the root of the checked-out repository.
 
 4.  To load and use the extension locally in your browser...
     -   **Firefox:** either:
@@ -132,13 +127,13 @@ Some further info on the test/build process:
 
 -   Automated tests are run as a pre-requisite part of the build process; you can also run them with `npm test`.
 
--   You can remove the `build/<browser>/` directories and ZIP files with `npm run clean:<browser>` and `npm run clean:all`, as with the build scripts above.
+-   You can remove the `build/<browser>/` directories and ZIP files for all browsers with `npm run clean:builds`.
 
--   Because the process of rasterising the SVG to variously-sized PNGs is slow, the PNGs are cached so they only need to be re-generated when the SVG changes. You can clean out the cache with `npm run clean:cache`.
+-   Because the process of rasterising the SVG to variously-sized PNGs is slow, the PNGs are cached so they only need to be re-generated when the SVG changes. They are cached in the `build/png-cache/` directory.
 
 -   The `pre-commit` hook is used to ensure only code that passes tests is committed (it does this by running a build, which, in turn, runs the tests). [Husky](https://github.com/typicode/husky) manages this so that a build is run before you are asked for a commit message.
 
--   The `build:chrome:test` script is provided for making an alpha/beta/test build for Chrome, which is the same as a normal build, but the extension is retitled to "Landmarks (test version)". A separate extension listing is required for publishing test versions in the Chrome Web Store. For Firefox Add-ons, a version number such as "2.1.0beta1" can be used and the built package can be uploaded to the extension's beta channel.
+-   For advanced use, you can run the build script directly (which bypasses the tests, beware) with `node scripts/build.js --help`.
 
 ### Test pages