(feat): support custom jest config paths via --config

- --config will now be parsed shallow merged with the defaults,
  just like package.json.jest already is

- previously adding --config to tsdx test would result in jest
  outputting a usage prompt (I believe due to the second --config
  that's added internally) and then the somewhat cryptic
  "argv.config.match is not a function"
  - if --config is detected, it will be parsed, merged, and then
    deleted from argv so that this error doesn't occur anymore

src/index.ts

@@ -487,7 +487,7 @@ prog
   .describe(
     'Run jest test runner in watch mode. Passes through all flags directly to Jest'
   )
-  .action(async () => {
+  .action(async (opts: { config?: string }) => {
     // Do this as the first thing so that any code reading it knows the right env.
     process.env.BABEL_ENV = 'test';
     process.env.NODE_ENV = 'test';
@@ -508,12 +508,29 @@ prog
     };
 
     // Allow overriding with jest.config
-    const jestConfigExists = await fs.pathExists(paths.jestConfig);
-    if (jestConfigExists) {
-      const jestConfigContents = require(paths.jestConfig);
+    const defaultPathExists = await fs.pathExists(paths.jestConfig);
+    if (opts.config || defaultPathExists) {
+      const jestConfigPath = resolveApp(opts.config || paths.jestConfig);
+      const jestConfigContents = require(jestConfigPath);
       jestConfig = { ...jestConfig, ...jestConfigContents };
     }
 
+    // if custom path, delete the arg as it's already been merged
+    if (opts.config) {
+      let configIndex = argv.indexOf('--config');
+      if (configIndex !== -1) {
+        // case of "--config path", delete both args
+        argv.splice(configIndex, 2);
+      } else {
+        // case of "--config=path", only one arg to delete
+        const configRegex = /--config=.+/;
+        configIndex = argv.findIndex(arg => arg.match(configRegex));
+        if (configIndex !== -1) {
+          argv.splice(configIndex, 1);
+        }
+      }
+    }
+
     argv.push(
       '--config',
       JSON.stringify({