Misc improvements

[chenyan-dfinity]

• Change export command to export function
• load 'a.sh?' silence the error if a.sh doesn't exist. load can take an expression instead of fixed text.
• add __main function that can take arguments from CLI
• exec function
• replica_url function
• add registry and cycles_ledger canister id
• verbose mode -v. Without verbose mode, only output the result if the return type is text.

[ericswanson-dfinity]

A couple minor grammar fixes