Add max file size support

[RamanaReddy0M]

closes

• Needs max file size and max runtime flags/switches  #12

Added

   -ms, -max-size value  max export data size (kb, mb, gb, tb) (default mb)

[Mzack9999]

merge conflicts

[tarunKoyalwar]

• suggesting minor change to avoid invalid domains
• update flag description in README.md

[ehsandeep]

human readable input for -ms option, for example

-ms 1gb
-ms 100mb
-ms 1 # will be considered as 1 mb when no unit is provided.

[Mzack9999]

I'd suggest to add this as a field type directly in goflags, so that the parsing is transparent (with default to 1Mb as minimal unit) and we can reuse the logic in all tools:

func (flagSet *FlagSet) SizeVarP(...) *FlagData { ... }
func (flagSet *FlagSet) SizeVar(...) *FlagData { ... }

What do you think?