Implement #259: add formatter option 'break_before_filename'

[codeinred]

This PR adds support for a break_before_filename option on the formatter (see #259 for examples & more information).

The following functions have been added to the API of the library:

class formatter {
    // ...

    // When printing the stacktrace, inserts a linebreak prior after the symbol name,
    // and inserts enough whitespace to align the filename with the symbol name.
    formatter& break_before_filename(bool do_break = true);

    // Formats an individual frame, with the given amount of extra whitespace added before the filename
    // in the event of a linebreak. This is needed because oftentimes an individual frame is printed
    // _after_ some identifier, such as the index of the frame within the trace, and if this information
    // is not taken into account, the filename will be misaligned.
    std::string format(const stacktrace_frame&, bool color, size_t filename_indent) const;

    // Same as above, but to a std::ostream&
    void print(std::ostream&, const stacktrace_frame&, bool color, size_t filename_indent) const;
    
    // Same as above, but with std::FILE*
    void print(std::FILE*, const stacktrace_frame&, bool color, size_t filename_indent) const;
}

Unit tests were added in test/unit/lib/formatting.cpp.

[jeremy-rifkin]

LGTM thanks!