Add compiled-in extending library functionality; Add small C99 library

This implements my idea of additional libraries you can add into Sparkling at compile time.
To illustrate how it works by describing how one would add sqlite3 bindings:

1) Add <code>SQLITE3 ?= 0 # sqlite3 binding</code> at after the <code>C99 ?= 0</code> line
2) Add
```make
ifeq ($(SQLITE3), 1)
	MODS += sqlite3
	LIBS += -lsqlite3
	ADDITIONAL_OBJS += $(OBJDIR)/sqlite3/*.o
endif
```
before the <code>ifneq ($(READLINE), 0)</code> line
3) Create an src/sqlite3 folder
4) Put
```make
SOURCES = $(wildcard $(SRCDIR)/*.c)
OBJECTS = $(patsubst $(SRCDIR)/%.c, $(OBJDIR)/%.o, $(SOURCES))

all: $(SOURCES) $(OBJECTS)

$(OBJDIR)/%.o: $(SRCDIR)/%.c
	$(CC) $(CFLAGS) -o $@ $<

clean:
	$(RM) $(OBJECTS) .DS_Store $(SRCDIR)/.DS_Store $(OBJDIR)/.DS_Store

.PHONY: all install clean
```
into src/sqlite3/Makefile
5) Implement stuff in src/sqlite3/rtlb.c
6) Then create a function <code>void (SpnVMachine *)</code> which registers globals & functions in the VM.
7) Finally add an <code>\_\_attribute\_\_((constructor))</code> function that calls <code>spn_add_libld</code> with the above function to register it to be run after the stdlib was loaded.
8) Create documentation at doc/libsqlite3.md

If you run make with <code>SQLITE3 = 1</code> now, src/sqlite3/rtlb.c will get compiled and linked into libspn.a, libspn.so and the interpreter executable.