New extension: strike through with two tildes
disable strike-through extension by default
Add strike-through support in ODF output
Add strike-through support in LaTeX output
Add strike-through support for groff output

A groff macro is defined to strike through words enclosed in pairs of
`~~` when rendering to the Postscript device. The macro doesn't yield
suitable results when rendering to any device other than Postscript.

As the macro application requires that the string be placed on its own
line, leading dots have to be escaped that result from splitting strings
like `~~Hello~~. Goodbye` after the last `~~`. Without escaping, the
groff output would be

    .ST "Hello"
    . Goodbye

leading to a failure to process ". Goodbye".
Merge upstream branch 'master' into strike-extension
Rewrote Strike parser to avoid exponential blowup.
Merge branch 'upstream' into strike-extension
add instructions for strike-through extension
Do not print "\usepackage{}" line

The generated LaTeX output is understood to be the document body,
so it may not contain macros that belong into the preamble.
Merge pull request #12 from rekado/strike-extension

New extension: strike through with two tildes