Uses a help tokens array to replace tokens in help text

[nkakouros]

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cc: @mbland

The --help-filter option and the # Help filter comment is too much for the user for sth that they may need to implement in every command script they write. Also, it limits a bit some use cases, for instance using docopts together with go-script-bash.

This PR adds a new associative array _GO_HELP_TOKENS. Each item in the array is a token=>replacement pair and go-script-bash automatically performs any replacements when generating the help text. This is complementary to the --help-filter method, which is compatible with old bash versions.

[coveralls]

Coverage increased (+0.03%) to 95.031% when pulling b34c9d4 on nkakouros-forks:help-tokens into 292e080 on mbland:master.