Set seed to fully control being repeatable

[Puriney]

Problem

1. Both the random colors and discrete color will not be repeatable in a new R session, as described in issue #13.
2. Setting seed as issue #3 did cannot make the randomColor's results repeatable. This is because internally the randomColor.js has its own seed number and should be set accordingly.

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After this PR is merged, these will happen:

1. Both the randomColor and distinctColorPalette will generate repeatable colors.
2. The seed number used by those two functions will NOT interrupt the randomness in the global environment.

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Run the added test code in readme_demo/test_repeatable.R to feel this PR

library(scales)
library(randomcoloR)

c1 <- distinctColorPalette(k = 20, seed=12)
c2 <- randomColor(count=20, hue = 'red', luminosity = 'light', seed=2020)

message('Distinct colors are repeatable:')
print(c1)
message('The seed number in randomcolorR does not interrupt the global randomness')
print(rnorm(10))
message('Random colors are repeatable:')
print(c2)
message('The seed number in randomcolorR does not interrupt the global randomness')
print(rnorm(10))

pdf("~/repeatable.pdf", width = 7, onefile = T)
plot(1)
scales::show_col(c1)
scales::show_col(c2)
dev.off()

Result:

Distinct colors are repeatable:
 [1] "#D73AE1" "#E0BCD9" "#7ACDE5" "#DF67CD" "#D3E04D" "#D19E87" "#8249E2"
 [8] "#E3A244" "#7C74E1" "#AFC1DA" "#D8DA91" "#6EE9D6" "#D4E4D1" "#8A5598"
[15] "#D498DE" "#82A0E3" "#E1687B" "#709E86" "#88E299" "#7BE953"
The seed number in randomcolorR does not interrupt the global randomness
 [1]  0.6725051  0.1504643  0.8634863 -0.2982528  1.8013768 -1.2937986
 [7] -0.4185201  0.7720086 -0.1350407 -1.4384490
Random colors are repeatable:
 [1] "#f7b5ad" "#fccdc2" "#f79479" "#fca78d" "#f298bf" "#f7c0ad" "#fcc4dc"
 [8] "#f97caa" "#ef88a7" "#f49cbb" "#f9b1c8" "#fcc4d4" "#fc7e9d" "#ef8b9a"
[15] "#f49fb0" "#f9b3bf" "#fcc7ce" "#fc808d" "#f28c93" "#f7a6a3"
The seed number in randomcolorR does not interrupt the global randomness
 [1]  1.7064048  0.1825268 -0.2825259 -0.4292656 -0.5898879  0.3666581
 [7]  0.7370429 -1.0799768  1.6670583 -0.3806285
null device
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