binds: add account specific bindings

When using aerc for multiple accounts often bindings might differ
slightly between accounts. For example:

* Account A archives to one directory (:archive)
* Account B archives to monthly directories (:archive month)

Add account specific bindings to allow the user to add a "context" to a
binding group using a context specifier and a regular expression.

Currently the only context specifier is 'account'.

The regular expression is validated against the accounts loaded from
accounts.conf and the configuration fails to load if there are no
matches.

Contextual bindings are merged with global bindings, with contextual
bindings taking precedence, when that context is active.

Bindings are be configured using a generic pattern of
'view:context=regexp'. E.g.:

    # Globally Applicable Archiving
    [messages]
    A = :read<Enter>:archive<Enter>

    # Monthly Archiving for 'Mailbox' Account
    [messages:account=Mailbox$]
    A = :read<Enter>:archive month<Enter>

In the above example all accounts matching the regular expression will
archive in the monthly format - all others will use the global binding.

Signed-off-by: Jonathan Bartlett <jonathan@jonnobrow.co.uk>

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