increment minimum go version

1.13 is the version actually used in the tests.
rearrange and cleanup main.go
improve documentation
remove useless variable
rewrite graph to scale the data instead of the graph
improve getDuration documentation
refactor graph function names
use terminal colors instead of unicode symbols
print a graph in terminal mode
cleanup display function names
print information about the next tide for term mode
show if tide is rising or falling
create a constant for the timezone
split display functions into display.go
ignore data folder