Add support for ClosureBuilder JVM options

The latest ClosureBuilder supports options for JVM flags, such as increasing the Java heap limits of the process.

This pull requests adds support for those flags, supplied as either a single string or an array of strings.

```bash
$ python closure/bin/build/closurebuilder.py --help
Usage: Utility for Closure Library dependency calculation.

ClosureBuilder scans source files to build dependency info.  From the
dependencies, the script can produce a manifest in dependency order,
a concatenated script, or compiled output from the Closure Compiler.

Paths to files can be expressed as individual arguments to the tool (intended
for use with find and xargs).  As a convenience, --root can be used to specify
all JS files below a directory.

usage: closurebuilder.py [options] [file1.js file2.js ...]


Options:
  -h, --help            show this help message and exit
  -i INPUTS, --input=INPUTS
                        One or more input files to calculate dependencies for.
                        The namespaces in this file will be combined with
                        those given with the -n flag to form the set of
                        namespaces to find dependencies for.
  -n NAMESPACES, --namespace=NAMESPACES
                        One or more namespaces to calculate dependencies for.
                        These namespaces will be combined with those given
                        with the -i flag to form the set of namespaces to find
                        dependencies for.  A Closure namespace is a dot-
                        delimited path expression declared with a call to
                        goog.provide() (e.g. "goog.array" or "foo.bar").
  --root=ROOTS          The paths that should be traversed to build the
                        dependencies.
  -o OUTPUT_MODE, --output_mode=OUTPUT_MODE
                        The type of output to generate from this script.
                        Options are "list" for a list of filenames, "script"
                        for a single script containing the contents of all the
                        files, or "compiled" to produce compiled output with
                        the Closure Compiler.  Default is "list".
  -c COMPILER_JAR, --compiler_jar=COMPILER_JAR
                        The location of the Closure compiler .jar file.
  -f COMPILER_FLAGS, --compiler_flags=COMPILER_FLAGS
                        Additional flags to pass to the Closure compiler. To
                        pass multiple flags, --compiler_flags has to be
                        specified multiple times.
  -j JVM_FLAGS, --jvm_flags=JVM_FLAGS
                        Additional flags to pass to the JVM compiler. To pass
                        multiple flags, --jvm_flags has to be specified
                        multiple times.
  --output_file=OUTPUT_FILE
                        If specified, write output to this path instead of
                        writing to standard output.
```