libcmd/markdown: handle allocation errors in lowdown_term_rndr

We upgrade to lowdown 0.8.0 [1] which contains a fix/improvement to a
behavior mentioned in this issue thread [2] where a big part of
lowdown's API would just call exit(1) on allocation errors since that
is a satisfying behavior for the lowdown binary.

Now lowdown_term_rndr returns 0 if an allocation error occurred which we
check for in libcmd/markdown.cc.

Also the extern "C" { } wrapper around lowdown.h has been removed as it
is not necessary.

[1]: https://github.com/kristapsdz/lowdown/blob/6ca7c855a063d1c77ae0b89405047cc3913a74d8/versions.xml#L987-L1006
[2]: https://github.com/kristapsdz/lowdown/issues/45#issuecomment-756681153