#65 fn: IMAGINARY, IMARGUMENT, IMCONJUGATE and IMDIV

Go 1.15 and later required, #65 fn: IMABS, IMCOS, IMCOSH, IMCOT, IMCSC, IMCSCH, IMEXP, IMLN and IMLOG10

#65 formula function: COMPLEX

typo fixed

This improves compatibility for absolute XML path, Windows-style directory separator and inline namespace;

This closes #787, avoid duplicate rich text string items, new formula fn: BIN2DEC, BIN2HEX, BIN2OCT, HEX2BIN, HEX2DEC, HEX2OCT, OCT2BIN, OCT2DEC, OCT2HEX

ref: #756, set cell as blank when SetCellValue with nil #756, new formula fn: BITAND, BITLSHIFT, BITOR, BITRSHIFT, BITXOR

This closes #785, support to change tab color; new formula function: FISHER, FISHERINV, GAMMA, GAMMALN, MIN, MINA, PERMUT

lint issue fixed and new formula function: ATAN, AVERAGE, AVERAGEA, CONCAT, CONCATENATE, COUNT, COUNTBLANK, MAX

This closes #752, fix incorrectly merged cells on duplicate row, and new formula function: LOWER, PROPER, UPPER