Ability to override compiler options without specifying tsconfig

[hellosmithy]

• Issue

Previously this module supported an inline tsconfig via __TS_CONFIG__. Now the only way to specify a custom override for module: 'commonjs' (which is a common use case for jest / typescript projects) is to provide a custom tsconfig file via tsConfigFile.

My use case is that I'm breaking out my jest config into a preset and as it shouldn't have any knowledge of the tsconfig for the consuming code, I want to be able to fallback to the defaultTSConfigFile as per

ts-jest/src/utils.ts

Lines 90 to 103 in f71f85e

	function getTSConfigPathFromConfig(globals: ConfigGlobals): string {
	const defaultTSConfigFile = getPathToClosestTSConfig();
	if (!globals) {
	return defaultTSConfigFile;
	}
	const tsJestConfig = getTSJestConfig(globals);
	if (tsJestConfig.tsConfigFile) {
	return tsJestConfig.tsConfigFile;
	}
	return defaultTSConfigFile;
	}

but just override the compiler module settings as you might do via the command line with the --module flag.

• Expected behavior

Ability to add compiler option overrides without specifying a custom tsconfig file https://www.typescriptlang.org/docs/handbook/compiler-options.html

For example:

"ts-jest": {
    "compilerOptions": {
        "module": "commonjs"
    }
}

Which could be deep merged into the settings found in defaultTSConfigFile.

I'm happy to submit a PR if the suggestion is acceptable.

• Output from your debug log
  n/a

• Link to a minimal repo that reproduces this issue

n/a

• Optional (but highly recommended) - Configure Travis (or your favorite system) with the minimal repo

n/a

[huafu]

@hellosmithy you can try the beta version (see #697) which allow you to define some inline options in the jest config: https://github.com/kulshekhar/ts-jest/tree/beta#typescript-configuration