Refactor functions to take the configuration as a parameter.
This allows setting a config option for a single function call, whereas
setting `openpgp.config` could lead to concurrency-related issues when
multiple async function calls are made at the same time.
`openpgp.config` is used as default for unset config values in top-level
functions.
`openpgp.config` is used as default config object in low-level functions
(i.e., when calling a low-level function, it may be required to pass
`{ ...openpgp.config, modifiedConfig: modifiedValue }`).
Also,
- remove `config.rsaBlinding`: blinding is now always applied to RSA decryption
- remove `config.debug`: debugging mode can be enabled by setting
`process.env.NODE_ENV = 'development'`
- remove `config.useNative`: native crypto is always used when available
Make all `read*` functions accept an options object, so that we can add config
options to them later (for #1166). This is necessary so that we can remove the
global `openpgp.config`, which doesn't work that well when importing
individual functions.
Furthermore, merge `readMessage` and `readArmoredMessage` into one function,
et cetera.
