More configuration options for liblsl library resolution

[tstenner]

Tangentially related to #6, I'd like an option to load either a specific library (as absolute path, e.g. /foo/bar/liblsl.so, a libname to be resolved by the system linker / in LD_LIBRARY_PATH (e.g. lsl or lsl64) or the standard behavior (look for an included liblsl).

The option could then be either set as environment variable (like MPLBACKEND for matplotlib) or as constant before loading pylsl (PYLSL_LIB='/foo/liblsl.so'; import pylsl).