Add indent option

[daguej]

This adds optional support for indentation of content rendered under headings.

With markdown.use(terminal, { indent: ' ' }) (two spaces):

Markdown	Rendered
# Heading 1 ## Heading 2 Some stuff indented twice # Only indented once	Heading 1 Heading 2 Some stuff indented twice Only indented once

If the indent option is not specified, there should be no change to rendered content.

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Bonus bugfix: While working on this feature, I discovered a bug in the way the library handles options. At instantiation, any options passed in overwrite the internal default options because _.merge(a, b) mutates a. This means that subsequent instantiations unexpectedly use options passed in previously.

a975738 adds a failing test that demonstrates the issue. fc7f464 fixes the bug, and also fixes a test that was mistakenly relying on this behavior.

[daguej]

@trabus thoughts?

[trabus]

@daguej Sorry for the delay, this looks great! Thanks for the PR!