Generate filesize types using flow

```
flowgen ./node_modules/filesize/filesize.d.ts -o 
./flow-libs/filesize.js.flow --add-flow-header --no-module-exports
```

flow-libs/filesize.js.flow

@@ -0,0 +1,123 @@
+/**
+ * Flowtype definitions for filesize
+ * Generated by Flowgen from a Typescript Definition
+ * Flowgen v1.13.0
+ * @flow
+ */
+
+declare var fileSize: Filesize$Filesize;
+declare export default typeof fileSize;
+declare var Filesize: typeof npm$namespace$Filesize;
+
+declare var npm$namespace$Filesize: {|
+  SiJedecBits: Class<Filesize$SiJedecBits>,
+  SiJedecBytes: Class<Filesize$SiJedecBytes>,
+  Options: Class<Filesize$Options>,
+  Filesize: Class<Filesize$Filesize>,
+|};
+declare interface Filesize$SiJedecBits {
+  b?: string;
+  Kb?: string;
+  Mb?: string;
+  Gb?: string;
+  Tb?: string;
+  Pb?: string;
+  Eb?: string;
+  Zb?: string;
+  Yb?: string;
+}
+
+declare interface Filesize$SiJedecBytes {
+  B?: string;
+  KB?: string;
+  MB?: string;
+  GB?: string;
+  TB?: string;
+  PB?: string;
+  EB?: string;
+  ZB?: string;
+  YB?: string;
+}
+
+declare type Filesize$SiJedec = Filesize$SiJedecBits &
+  Filesize$SiJedecBytes & {
+    [name: string]: string,
+    ...
+  };
+
+declare interface Filesize$Options {
+  /**
+   * Number base, default is 2
+   */
+  base?: number;
+
+  /**
+   * Enables bit sizes, default is false
+   */
+  bits?: boolean;
+
+  /**
+   * Specifies the SI suffix via exponent, e.g. 2 is MB for bytes, default is -1
+   */
+  exponent?: number;
+
+  /**
+   * Enables full form of unit of measure, default is false
+   */
+  fullform?: boolean;
+
+  /**
+   * Array of full form overrides, default is []
+   */
+  fullforms?: string[];
+
+  /**
+   * BCP 47 language tag to specify a locale, or true to use default locale, default is ""
+   */
+  locale?: string | boolean;
+
+  /**
+   * ECMA-402 number format option overrides, default is "{}"
+   */
+  localeOptions?: Intl.NumberFormatOptions;
+
+  /**
+   * Output of function (array, exponent, object, or string), default is string
+   */
+  output?: "array" | "exponent" | "object" | "string";
+
+  /**
+   * Decimal place, default is 2
+   */
+  round?: number;
+
+  /**
+   * Decimal separator character, default is `.`
+   */
+  separator?: string;
+
+  /**
+   * Character between the result and suffix, default is ` `
+   */
+  spacer?: string;
+
+  /**
+   * Standard unit of measure, can be iec or jedec, default is jedec; can be overruled by base
+   */
+  standard?: "iec" | "jedec";
+
+  /**
+   * Dictionary of SI/JEDEC symbols to replace for localization, defaults to english if no match is found
+   */
+  symbols?: Filesize$SiJedec;
+
+  /**
+   * Enables unix style human readable output, e.g ls -lh, default is false
+   */
+  unix?: boolean;
+}
+
+declare interface Filesize$Filesize {
+  (bytes: number, options?: Filesize$Options): string;
+  partial: (options: Filesize$Options) => (bytes: number) => string;
+}