Add readline support to cli

This PR adds readline support to the daemon and monero-wallet-cli. Only
GNU readline is supported (e.g. not libedit) and there are cmake checks
to ensure this.

There is a cmake variable, Readline_ROOT_DIR that can specify a
directory to find readline, otherwise some default paths are searched.

There is also a cmake option, USE_READLINE, that defaults to ON. If set
to ON, if readline is not found, the build continues but without
readline support.

One negative side effect of using readline is that the color prompt in
the wallet-cli now has no color and just uses terminal default. I know
how to fix this but it's quite a big change so will tackle another time.

contrib/epee/src/CMakeLists.txt

@@ -26,7 +26,12 @@
 # STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-add_library(epee STATIC hex.cpp http_auth.cpp mlog.cpp string_tools.cpp)
+if (USE_READLINE AND GNU_READLINE_FOUND)
+  add_library(epee STATIC hex.cpp http_auth.cpp mlog.cpp string_tools.cpp readline_buffer.cpp)
+else()
+  add_library(epee STATIC hex.cpp http_auth.cpp mlog.cpp string_tools.cpp)
+endif()
+
 # Build and install libepee if we're building for GUI
 if (BUILD_GUI_DEPS)
     if(IOS)