develop more generic "update" class for long-tail of version replacements

[bcoe]

Looking at google-api-java-client, there was a long tail of files we failed to update the version in:

./google-api-client/src/test/java/com/google/api/client/googleapis/GoogleUtilsTest.java:    String version = "1.30.3";
./google-api-client/src/test/java/com/google/api/client/googleapis/GoogleUtilsTest.java:    String version = "1.30.3-SNAPSHOT";
./google-api-client/src/main/java/com/google/api/client/googleapis/GoogleUtils.java:  public static final String VERSION = "1.30.3".toString();
./google-api-client-bom/README.md:      <version>1.30.3</version>

We continue to add one off fixes, like #304, but it might be nice to have a slightly more generic update class that handles a variety of simple string replacements.

e.g., <version>0.3.0</version>, VERSION = "0.20.4", maybe the class could support a variety of regexes like releasetool.