Allow adding build number to compiled gem version

[luislavena]

Introduce an optional --build-number compile option to increase the compiled gem version from the original one.

This is useful since local compilation might be trial and error about compilation options and gems pushed to RubyGems or Gemstash are immutable and cannot be replaced.

With this change, you can append this build number to the version and remain compatible with any existing gem requirement like ~> X.Y or ~> X.Y.Z.

Imagine compiling nokogiri 1.11.3 using system libraries:

$ gem compile nokogiri-1.11.3.gem --prune -- --use-system-libraries
...

Then, realizing that your OS system libraries are vulnerable to a reported CVE, you can build and push a compatible 1.11.3 version that uses the bundled dependencies instead:

$ gem compile nokogiri-1.11.3.gem --prune --build-number 1
Unpacking gem: 'nokogiri-1.11.3' in temporary directory...
Building native extensions. This could take a while...
...
    Successfully built RubyGem
    Name: nokogiri
    Version: 1.11.3.1
    File: nokogiri-1.11.3.1-x86_64-linux.gem

This new version 1.11.3.1 will satisfy most of pessimistic dependency constraints:

v1 = Gem::Version.create("1.11.3")
# => #<Gem::Version "1.11.3">

v2 = Gem::Version.create("1.11.3.1")
# => #<Gem::Version "1.11.3.1">

r1 = Gem::Requirement.create("~> 1.11.3")
r2 = Gem::Requirement.create("~> 1.11")

r1.satisfied_by? v1
# => true

r1.satisfied_by? v2
# => true

r2.satisfied_by? v1
# => true

r2.satisfied_by? v2
# => true

Giving you build flexibility and remain compatible.

Closes #57
Closes #59