EHN add navigator option in plot()

False provides only slider and and external Spectrum/Image can be provided
Merge branch 'master' into ENH_plot_external_navigator

Conflicts:
	hyperspy/signal.py
Merge branch 'master' into ENH_plot_external_navigator
Merge branch 'master' into ENH_plot_external_navigator
Merge branch 'master' into ENH_plot_external_navigator
Solved bug link to axes_manager
Merge branch 'master' into ENH_plot_external_navigator

Conflicts:
	hyperspy/drawing/mpl_hse.py
Merge branch 'master' into ENH_plot_external_navigator
EHN: Add 'Spectrum' option
EHN: spectrum not Spectrum
BUG: solved a bug link to 'spectrum'
EHN: Dynamic navigator vs fixed navigator
EHN: write documentation
EHN: add reference
Merge branch 'master' into ENH_plot_external_navigator

Conflicts:
	hyperspy/signal.py
Merge branch 'master' into ENH_plot_external_navigator
Merge branch 'master' into ENH_plot_external_navigator
EHN: merge
rename .PNG -> .png
Merge branch 'ENH_plot_external_navigator' of https://github.com/hyperspy/hyperspy into ENH_plot_external_navigator