Allow passing a custom demangler

[fperrin]

Beyond C++, other language compilers may also mangle names, and need to use a custom demangler. This is done with a new --demangler option, defaulting to c++filt when --demangle is given. This is compatible with the current CLI, but the API changed a little bit (LcovCobertura takes a string for demangler instead of a boolean for demangle).