Configurable params

[mikemorris]

Refs #51 and #38 (comment)

Tracking in configurable branch, currently stuck at https://github.com/mapbox/node-fontnik/blob/ed5cc2727bb4014211c917e7e07385389c834891/src/fontnik/glyphs.cpp#L38-40

../src/fontnik/glyphs.cpp:38:40: error: no matching member function for call
      to 'get'
    const std::string fontstack(params.get(ParamKey::Fontstack));
                                ~~~~~~~^~~
../include/fontnik/params.hpp:55:21: note: candidate template ignored:
      couldn't infer template argument 'T'
    inline const T *get(ParamKey key) const {
                    ^
../src/fontnik/glyphs.cpp:39:36: error: no matching member function for call
      to 'get'
    const std::string range(params.get(ParamKey::Range));
                            ~~~~~~~^~~
../include/fontnik/params.hpp:55:21: note: candidate template ignored:
      couldn't infer template argument 'T'
    inline const T *get(ParamKey key) const {
                    ^
../src/fontnik/glyphs.cpp:40:46: error: no matching member function for call
      to 'get'
    const std::vector<uint32_t> chars(params.get(ParamKey::Chars));
                                      ~~~~~~~^~~
../include/fontnik/params.hpp:55:21: note: candidate template ignored:
      couldn't infer template argument 'T'
    inline const T *get(ParamKey key) const {
                    ^

[springmeyer]

no longer an issue after #80 is merged.