Simplify sharing contexts at top level

If we have a shared context defined in one file:

```
def some_shared_context(context):
@context.before
def before(self):
pass
```

and we want to use it at different files, we need to do this:

```
from wherever import some_shared_context

@context
def first(context):
context.shared_context(some_shared_context)
context.merge_context("some shared context")
```
and on the other file:
```
from wherever import some_shared_context

@context
def second(context):
context.shared_context(some_shared_context)
context.merge_context("some shared context")
```

We can somewhat easily modify the `merge_context` function to accept the function directly, simplifying things:

```
from wherever import some_shared_context

@context
def first(context):
context.merge_context(some_shared_context)
```
and on the other file
```
from wherever import some_shared_context

@context
def second(context):
context.merge_context(some_shared_context)
```