Switch to Lets-Plot Kotlin API 0.0.18-SNAPSHOT

Added:
 - Geometries:
   - geom_crossbar()
   - geom_errorbar()
   - geom_freqpoly()
   - geom_linerange()
   - geom_pointrange()
   - geom_polygon()
   - geom_ribbon()
   - geom_step()

 - Standardize input data series as List<*>:
   - Iterable<*>
   - Sequence<*>
   - Array<*>
   - ByteArray
   - ShortArray
   - IntArray
   - LongArray
   - FloatArray
   - DoubleArray
   - CharArray

 - Standardize values in the input data series:
   - Date, Instant and ZonedDateTime as the number of milliseconds from the epoch of 1970-01-01T00:00:00Z.
   - Char as string

Fixed:
 - geom_density:
   - defaults: alpha=0, fill=white
   - support weight aesthetic
 - geom_histogram: added 'bin' parameters: binCount, binWidth, center, boundary.
(cherry picked from commit f2ec22b)

libraries/lets-plot.json

@@ -1,7 +1,7 @@
 {
   "properties": {
     "libraryVersion": "1.4.2",
-    "apiVersion": "0.0.17-SNAPSHOT",
+    "apiVersion": "0.0.18-SNAPSHOT",
     "isolatedFrame": ""
   },
   "link": "https://github.com/JetBrains/lets-plot-kotlin",