Support C-method source in show_source command

[st0012]

The main challenge of this support comes from acquiring the C-method source files:

• Unless users install Ruby with ruby-install or ruby-build -k, Ruby's C source code wouldn't be preserved after Ruby is built
• Between ruby-install and ruby-build -k, the location of sources are different:
  • ruby-install: ~/src or /usr/local/src for the root user
  • ruby-build -k: ~/.cache
• Pry uses pry-doc to serve as an installable source database

So instead of relying on the installed CRuby source, downloading the source later may be a better approach (proposed by @k0kubun).

Questions:

• Should it be rdoc, irb, or another tool's responsibility to perform this action?
• Should it install multiple Ruby versions' sources, or just 1?
  • I prefer the former, like downloading all actively maintained Ruby versions at once
• TBD