Add :numref: role to refer figures, tables and code-blocks by its fignum

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extra : histedit_source : 3fa15470ffeb9d3d3124d030fa651e478b4d3e54%2C6fe4797c26d99eaf64868ef4a6c32d2098c0f497
Merge with default
Support :numref: in LaTeX writer
Support :numref: in other writers
Fix numfig got error if table without title exists
Update testcases for :numref:
Catch KeyError resolving numref if target docs does not included in toctree
Fix position of \label tag for table was invalid in LaTeX
Fix position of \label tag for literal_block was invalid in LaTeX
Merge with default
Add docs about confval for numfig
Update docs by reviewing
Add docs about :numref: role

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extra : histedit_source : 9ad64ad1ae1f5adb8c894d52f33888b102c3b645%2C80e5104a2de27358420711538482a74231d4323f
Merged in tk0miya/sphinx (pull request #303)

Add :numref: role to refer figures, tables and code-blocks by its fignum