fix(1337): change doom-1337-blue-modeline to doom-1337-purple-modeline

[bormoge]

This changes the name and behavior of doom-1337-blue-modeline; instead of showing a white modeline (base-8) it changes the color to a purple identical to doom-dark-+ (dark-violet).

Screenshots:

• doom-1337 modeline as it normally is.

• doom-1337 modeline with doom-1337-blue-modeline set to t.

• doom-1337 modeline with the new doom-1337-purple-modeline set to t.

Fix: #880

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