feat: pre-compile the typescript config

This compilation step, currently done inside filing-cabinet, causes a significant
slowdown for large trees.

Closes gh-114

Author: FauxFaux

lib/Config.js

@@ -22,6 +22,14 @@ class Config {
     if (!this.directory) { throw new Error('directory not given'); }
     if (this.filter && typeof this.filter !== 'function') { throw new Error('filter must be a function'); }
 
+    if ('string' === typeof this.tsConfig) {
+      debug('preparsing the ts config into an object for performance');
+      const ts = require('typescript');
+      const tsParsedConfig = ts.readJsonConfigFile(this.tsConfig, ts.sys.readFile);
+      const obj = ts.parseJsonSourceFileConfigFileContent(tsParsedConfig, ts.sys, path.dirname(this.tsConfig));
+      this.tsConfig = obj.raw;
+    }
+
     debug('given filename: ' + this.filename);
 
     this.filename = path.resolve(process.cwd(), this.filename);