Improve text/plain show for AbstractDataFrame

[bkamins]

These are things to consider when improving show for AbstractDataFrame:

• correct handling of wide columns (Clean up column omission when columns overflow IOContext width #2087)
• optionally change textual representation of missing (adding show() option missingstring to change the string that prints for missing values #1688)
• optionally remove type from heading (Optionally remove type from heading #2116)
• avoid printing vertical bars to have more compact printout
• right align numbers and make decimal dot . in one row (like arrays do)
• correct the display problem when printout width is exactly the same as terminal width (we should use "terminal width-1" to determine skipping printing columns)
• correctly handle values spanning multiple lines
• improve handling of very wide columns (maybe we should have some maximum width after which we clip the output like we now do for types), Dataframes with wide columns have poor plain/text representations #1272
• discuss whether names of all omitted columns should be printed at the end (like dplyr)
• fix circular reference bug (Circular reference in DataFrame bug #2135)

[bkamins]

@nalimilan - please add to the list if you have something more

[bkamins]

PrettyTables.jl now handles these.