Let figure file name and format be determined by file extension

[joa-quim]

No description provided.

[codecov-io]

Codecov Report

Merging #290 into master will increase coverage by <.01%.
The diff coverage is 98.73%.

@@            Coverage Diff             @@
##           master     #290      +/-   ##
==========================================
+ Coverage   93.81%   93.82%   +<.01%     
==========================================
  Files          80       80              
  Lines        3639     3642       +3     
==========================================
+ Hits         3414     3417       +3     
  Misses        225      225

Impacted Files	Coverage Δ
src/pswiggle.jl	100% <100%> (ø)	⬆️
src/common_options.jl	96.03% <100%> (+0.01%)	⬆️
src/pssolar.jl	100% <100%> (ø)	⬆️
src/gmtlogo.jl	94.28% <100%> (-0.31%)	⬇️
src/grdview.jl	97.22% <100%> (ø)	⬆️
src/pstext.jl	100% <100%> (ø)	⬆️
src/grdcontour.jl	100% <100%> (ø)	⬆️
src/grdimage.jl	100% <100%> (ø)	⬆️
src/psclip.jl	100% <100%> (ø)	⬆️
src/pshistogram.jl	100% <100%> (ø)	⬆️
... and 11 more

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